Short Answer - Not Really
International treaties used Great Britain to refer to the whole state up until the early 20th Century and the term, and its French language equivalent Grande Bretagne is to this day widely used by British officials - example - but I think that falls short of a precise legal definition particularly as it is sometimes unclear whether Great Britain is being used to mean just the UK - the state recognised in international law - or the UK plus Mann and the Channel Islands which the UK sometimes enters into treaties on behalf of.
Whenever Great Britain is used in statutes of the UK Parliament it appears to be using the term as a geographical name for the island of Great Britain (plus or minus the islands on its insular shelf). Some people think they detect in statutes passed in 1800, 1927 and 1978 an implication that some kind of political entity exists whose territory is the island of Great Britain plus the islands on its insular shelf - i.e. some kind of political left-over from the 18th Century Kingdom of Great Britain but I think this reading of those statutes is incorrect.
So I conclude that whilst Great Britain is widely used in a political sense to mean the British Islands as a whole, it does not have a precise legal meaning (beyond the ordinary geographical meaning as the name of an island).
International treaties used Great Britain to refer to the whole state up until the early 20th Century. Often Great Britain is in the title of the treaty as the name of the state without the words United Kingdom of Great Britain and Ireland appearing at all - see here, here, here, here, here and here. Where the words United Kingdon of Great Britain and Ireland do appear in a treaty that is usually because the treaty has been entered into by the monarch herself - Her Britannic Majesty - and the words appear as part of her title (along with Empress of India) - see here, here and here.
Even where United Kingdom of Great Britain and Ireland is indicated as the contracting party in a treaty, when the signed treated is presented to Parliament in a command paper the short title assigned will often be Treaty between Great Britain and... as in this example so for a very long time Parliament has been using Great Britain to mean not just England, Wales and Scotland but the entire state.
Treaties entered into during the time of the Irish War of Independence refer to the contracting party as Great Britain with no mention of any part of Ireland at all - example and those entered into immediately after the Anglo-Irish treaty - when the final outcome was unclear - may simply United Kingdom as the name of the contracting state - see here.
Up until about 1953 sometimes treaties were entered into in the name of Great Britain and Northern Ireland (example) and sometimes in the name of the United Kingdom of Great Britain and Northern Ireland (example) but as far as I can see United Kingdom of Great Britain and Northern Ireland is used consistently after 1953.
So Great Britain was the everyday term used by the government for the UK, or possibly for all the British Islands, but I am not sure that really counts as a precise legal definition.
Act of Union 1800
By the Acts of Union 1800 the Kingdom of Great Britain and the Kingdom of Ireland united to become the Kingdom of Great Britain and Ireland. Some people see in the name Kingdom of Great Britain and Ireland an indication that there is a extant political entity Great Britain whose territory is the same as the previous Kingdom of Great Britain but it is difficult to see what such a political entity could actually be. Great Britain has never been the name of a legal jurisdiction and with the creation of the UK, the Parliament of Great Britain, responsible only for the island of Great Britain (and its insular shelf) ceased to exist as a sperate entity.
I believe the flaw in this theory is the assumption that if a political entity is called A and B then A and B must themselves be political subdivisions of the entity. In fact it it extremely common for a state to name itself after an island or more than one island and the names of the islands do not thereby become separate political entities. The state known as St Vincent and the Grenadines has the names of two islands (or rather an island and an island range) in its title but that does not thereby make the Grenadines a separate political entity. St Vincent is in fact used as a political term but only as a short title for the whole state not as a political sub-division.
Royal and Parliamentary Titles Act 1927
This Act was passed after the creation of the Irish Free State as a Dominion outside the UK so that the territory of the UK was reduced. The first section authorises the King to change his style by proclamation. Section 2 deals with the title of Parliament:-
2 Alteration of the style of Parliament.
(1) Parliament shall
hereafter be known as and styled the Parliament of the United Kingdom
of Great Britain and Northern Ireland; and accordingly, the present
Parliament shall be known as the Thirty-fourth Parliament of the
United Kingdom of Great Britain and Northern Ireland, instead of the
Thirty-fourth Parliament of the United Kingdom of Great Britain and
(2) In every Act passed and public document issued after the
passing of this Act the expression " United Kingdom " shall, unless
the context otherwise requires, mean Great Britain and Northern
Some people suggest that sub-section 2(2) defines a political entity named Great Britain distinct from, but co-terminous with, the island of Great Britain but surely, in context, the purpose of s2(2) is much simpler. Parliament is changing its name from the Parliament of the United Kingdom of Great Britain and Ireland to the Parliament of the United Kingdom of Great Britain and Northern Ireland. Section 2(2) simply says that where the simple two word phrase United Kingdom is used then unless the context otherwise requires it is assumed to mean the United Kingdom of Great Britain and Northern Ireland rather than the United Kingdom of Great Britain and Ireland
The government minster sponsoring the Bill in parliament made clear its limited purpose:
The Government have decided at the same time to make provision for a
change in the title of Parliament, which was enacted by the Act for
the Union of Great Britain with Ireland under the style of "Parliament
of the United Kingdom of Great Britain and Ireland." For the reasons
which I have already given the use of the expression "United Kingdom
of Great Britain and Ireland" is no longer appropriate. The Irish Free
State is not represented in the same Parliament as Great Britain while
Northern Ireland, in addition to being represented in this Parliament,
is subject in many matters to its legislative authority and has
remained in all respects an integral part of the United Kingdom. The
Government are advised by their appropriate advisers in such matters
that it would be in accordance with the constitutional position that
Parliament should henceforth be known as the Parliament of the United
Kingdom of Great Britain and Northern Ireland. That description has
the advantage of corresponding with the facts. The Bill does not in
any way, of course, affect the constitution of Parliament but merely
alters its name to bring it into correspondence with the facts. I beg
to move that this Bill be now read a second time.
Some people argue that in the 1927 Act Northern Ireland cannot refer to a geographical extent because it is inaccurate (Northern Ireland does not actually include all of the North of the island) and so it must be used as a political term (the name of a legal jurisdiction) and so Great Britain must also be being used in a political rather than a geographical sense. But in the context of the 1921 Anglo Irish Treaty it makes perfect sense to use the term Northern Ireland to define a geographical extent.
Under the treaty the Irish Free State was created as a new Dominion whose territory was the whole island of Ireland with its parliament in Dublin and a devolved Parliament in Belfast. Under the terms of the treaty the Northern Ireland Parliament could opt out and re-join the UK - which it did - and thereafter a joint Boundary Commission would determine the boundary of Northern Ireland. The work of the Boundary Commission was delayed by the Irish Civil War and and from the start it was controversial. It did not report until 1925 and, in the event its findings proved to be so politically sensitive that all parties agreed in December 1925 not to implement it but to agree that the initial provisional boundary should instead also be the final boundary:
1.—The powers conferred by the proviso to Article 12 of the said Articles of Agreement on the Commission therein mentioned are hereby
revoked, and the extent of Northern Ireland for the purposes of the
Government of Ireland Act, 1920, and of the said Articles of Agreement
shall be such as was fixed by sub-section (2) of section one of that
The agreement was formally registered with the League of Nations on 8 February 1926.
Thus, with that background, Northern Ireland was, in 1927, understood as being a very precisely defined geographical area and it makes sense that the term would be used, together with Great Britain to indicate the reduced territory of the United Kingdom.
Interpretation Act 1978
This Act simply repeals the words "Act passed" in section 2(2) of the 1927 Act and re-enacts the definition along with other general definitions
"United Kingdom" means Great Britain and Northern Ireland.
So this is simply a tiding up measure. Previously the default meaning of United Kingdom in both Acts and public documents was defined in the 1927 Act. Now the definition for public documents remains in the 1927 Act but the definition for Acts moves to the Interpretation Act 1978 along with all the other default definitions applying to Acts. The actual default definition is unchanged.
There is also the point - albeit rather a technical point - that only United Kingdom is defined by the IA 1978. Great Britain even if it were used in a political sense in IA 1978 would still not actually be defined by IA 1978 as a default definition for the future.
An Interpretation Act, like all Acts, must itself be interpreted but there is an important distinction between:
What a word - any word - means within the particular Interpretation Act
The terms which are actually defined by the Interpretation Act to be default meanings for future Acts.
Take an example (this example works in British English - I hope it works in other varieties of English as well)...
The word cash can mean
A. coins (rather than banknotes)
B. coins and banknotes (rather than bank deposits)
C. coins, banknotes, and bank deposits (rather than gold bars or shares)
If, in an Interpretation Act, it is says
"money" means cash and bank deposits
Then "money" is defined as meaning that, by default, in future Acts (point 2 above).
When understanding (interpreting) the Interpretation Act itself - point 1 above - you go through the process of working out what cash means and that is not difficult - it obviously has meaning B. But the fact that cash has meaning B in the Interpretation Act and is used in the process of defining a term (money) as a term with a default meaning for the future, does not, of course, mean that cash itself is defined for the future as always having meaning B. If cash is used in future it could have any of the meanings A, B or C depending on context.
So when the Interpretation Act 1978 says
"United Kingdom" means Great Britain and Northern Ireland.
That defines "United Kingdom" with a default meaning for the future, but it does not define "Great Britain" for the future.
It seems clear that Great Britain in the above sentence is being used in a geographical sense but even if, hypothetically, it were being used in some kind of political sense to mean a political entity co-terminous with the geographical island of Great Britain and its insular shelf, such a usage would not constitute a legal definition for the future.
So I would conclude that whilst Great Britain is widely used in a political sense to mean the British Islands, it does not have a precise legal meaning (beyond the ordinary geographical meaning as the name of an island).