What act or legal document specifically defines an "Act of Parliament" as a federal act, or primary act under the government of Canada, and not for example a provincial parliament?
owjburnham's answer is very good, but there is an actual definition (if slightly circular) for Parliament in the Interpretation Act (R.S.C., 1985, c. I-21):
s. 35 (1) In every enactment, [...] Parliament means the Parliament of Canada; (Parlement)
Act is defined in the same section as referring to provincial legislation.
This of course only applies to federal legislation; provincial Interpretation Acts don't seem to define it.
I fear that it may mostly be defined by common sense and context, rather than any particular statute. I've certainly not been able to find anything quite as explicit as, for example:
An Act of Parliament is an Act passed by the Parliament of Canada and definitely not in any way an Act of a provincial legislature.
Nevertheless, if you'll bear with me while I slog through a sea of clauses that all imply the above, then the best places to look for usage and definition are the Constitution Acts, 1867 and 1982, as they form (the basis/bulk of) the Canadian Constitution. I've also (credit to Zizou212) included some definitions from the Canadian Criminal Code.
Looking at the Constitution Act 1982 (as amended, via the Canadian justice department's website) and taking the crudest possible approach (i.e. looking for instances of "Act of") there is, in the main body of the text, only one reference to Acts of Parliament, or to Acts of the provincial legislatures, namely in the notwithstanding clause (section 33 of the Charter of Rights and Freedoms):
Parliament or the legislature of a province may expressly declare in an Act of Parliament or of the legislature, as the case may be, that the Act or a provision thereof shall operate notwithstanding a provision included in section 2 or sections 7 to 15 of this Charter.
Not, I fear, not a full answer to your question, but it's worth noting that a distinction is drawn between "Parliament" and "the legislature of a province".
For context, section 32, directly above, reads,
This Charter applies (a) to the Parliament and government of Canada in respect of all matters within the authority of Parliament [...]; and (b) to the legislature and government of each province in respect of all matters within the authority of the legislature of each province.
We're getting closer. I would argue (though IANAL) that from these two sections alone it's pretty clear that Acts of Parliament are Acts passed by the Parliament of Canada ("This Charter applies [...] to the Parliament and government of Canada") while Acts of "the legislature" of each province are Acts passed by the legislature of that province ("This Charter applies [...] to the legislature and government of each province").
Going back to the original question, I think it's pretty clear that "Parliament" – with a capital P – is (fairly) explicitly the Parliament of Canada. Note that it's the only body of the 4 mentioned there that gets a capital letter, and I imagine that it's because it's the only one referred to by its proper name. Whereas "the legislature and government of each province" is, presumably, just a common sense descriptor and a stand-in for the proper names of those governments and legislatures – it would be tedious to say "the Legislative Assembly of Ontario, and the National Assembly of Quebec, and [...]" – while "the government of Canada" is, I assume, not capitalised because it's also just a descriptor (the 'proper' name being Her Majesty's Government) but I'm descending now entirely into the realms of wildest speculation, as the Constitution Act 1867 is perfectly happy to use the phrase "Government of Canada" (though those were, seemingly, more capital-heavy times).
In this Act, Act includes
(a) an Act of Parliament,
(b) an Act of the legislature of the former Province of Canada,
(c) an Act of the legislature of a province, and
(d) an Act or ordinance of the legislature of a province, territory or place in force at the time that province, territory or place became a province of Canada;
Again, the mention of "Parliament" alongside – and distinct from – provincial legislatures, makes fairly clear that it's the Parliament of Canada.
Going back to the typesetter's nightmare that is the 1867 Act, we can eke out a few more puzzle pieces in the definitions:
In the Constitution Act, 1867, Part IV:
There shall be One Parliament for Canada, consisting of the Queen, an Upper House styled the Senate, and the House of Commons [...] The privileges, immunities, and powers to be held, enjoyed, and exercised by the Senate and by the House of Commons, and by the members thereof respectively, shall be such as are from time to time defined by Act of the Parliament of Canada
There shall be a Legislature for Ontario [...]
There shall be a Legislature for Quebec [...]
the Legislature of each of the Provinces of Nova Scotia and New Brunswick shall [...] continue as it exists at the Union
It shall be lawful for the Queen, by and with the Advice and Consent of the Senate and House of Commons, to make Laws for the Peace, Order, and good Government of Canada, in relation to all Matters not coming within the Classes of Subjects by this Act assigned exclusively to the Legislatures of the Provinces [...]
In each Province the Legislature may exclusively make Laws in relation to Matters coming within the Classes of Subjects next hereinafter enumerated
Between those three Acts, I'm hoping there are enough bits of context to make clear that Parliament only ever refers to the Parliament of Canada, and not to the provincial legislatures. If, however, you want one more bit of evidence, I can offer the pre-amble to the Canada Act, 1982 the Constitution Act's slightly older, British twin. This was the law that finally patriated the Canadian Constitution, passing (at the British Parliament in Westminster) an Act that defined how Canada could amend its own Constitution, and renouncing any power for the British Parliament to do the same. In that text, (emphasis mine) there are, by necessity, two Parliaments discussed and accordingly it's always made very clear which one is being talked about:
An Act to give effect to a request by the Senate and House of Commons of Canada
Whereas Canada has requested and consented to the enactment of an Act of the Parliament of the United Kingdom to give effect to the provisions hereinafter set forth and the Senate and the House of Commons of Canada in Parliament assembled have submitted an address to Her Majesty requesting that Her Majesty may graciously be pleased to cause a Bill to be laid before the Parliament of the United Kingdom for that Purpose.
Be it therefore enacted by the Queen's Most Excellent Majesty, by and with the advice and consent of the Lords Spiritual and Temporal, and Commons, in this present Parliament assembled, and by the authority of the same, as follows:
The Constitution Act, 1982 set out in schedule B to this Act is hereby enacted for and shall have the force of law in Canada and shall come unto force as provided in that Act.
No Act of the Parliament of the United Kingdom passed after the Constitution Act, 1982 comes into force shall extend to Canada as part of its law.
...ergo, in the main body Constitution Acts etc., "Parliament" was presumably felt to be clear enough.
Canada inherits the definition of an "Act" from English law.
An Act of Canada is a Bill passed by the parliament of Canada and signed into law by the Governor General.
An Act by province X is a Bill passed by the legislature of the province and signed into law by the (Lieutenant) Governor of that province.