As the Oxford PID says, annexation is the acquisition of land/territory from one state (=Souvreigh State, as in it's meaning of country, like the US, not a federal state like Texas) to another. That is the same what Wikipedia says.
It also says, that to annex territory, occupation is presupposed. Let's take two more clear examples:
- The incorporation of the Sudetenland by Nazi Germany via the Munich Agreemen was a multilateral contract of other countries but the Czech Republic to accept the demands. This allowed the Nazi Regime to occupy these areas without war or further international backlash and annex them.
- One can't say that the Federal Republic of Germany did annex the German Democratic Republic, as there was no occupation and it was written into international law by a multilateral contract as an act of union.
The Kashmir Question
Now, Kashmir is a powderkeg question. And the TL;DR here: No, India did not annex Kashmir and Jammu, Kashmir and Jammu did accede the Dominion of India.
Let's look at it historically, starting with 1846. Then, the Princely State of Kashmir and Jammu was created when the very ethnically diverse territories that the British east Indian Company annexed from the Sikh Empire as a result of the first Anglo-Sikh War were sold to Gulab Singh of Jammu with the Treaty of Amritsar.
In 1857 the Kashmeer sided with the British in the revolt. The British won and established the British Raj and assumed direct control over all the Indian subcontinent - but technically the Princely State of Kashmir and Jammu still existed as a part of the Raj. Time went by.
1947 comes around, the British have given the Raj some years to negotiate the landscape of the countries to release from their rule as a reward for their part in the second world war. The Indian Independence Act comes into reality, creating the Dominion of Pakistan and Dominion of India, with each Princely State having been given time to choose which to join beforehand. Only Hyderabad, Junagadh (and subsidiaries), the Khanate of Kalat and the Princely State of Jammu and Kashmir have not chosen by 15th of August 1947. Hyderabad joins India 2 months late, Junagadh Pakistan. Kalat signed a standstill with Pakistan but was independent otherwise until March of 1948 as the Khan discussed some terms with the Dominion of Pakistan. However, the Maharaja Hari Singh of Kashmir and Jammu had planned to join neither and stay independent. But time wouldn't allow that to endure...
The Princely State of Kashmir and Jammu tried to get both Dominions to sign a standstill agreement that would keep the Status Quo on all administrative things until a proper agreement was signed. The Dominion of Pakistan accepted while the Dominion of India did demand that they'd only sign if they had a delegate to discuss terms with. The Dominion of Pakistan then blockaded the Princely State, technically violating their standstill agreement. Hari Singh did reach out for the Dominion of India's help as a result and got essential goods from them as aid. By now it is September 1947. Kashmir is still independent.
In the Poonch District, an uprising happens under Muhammad Ibrahim Khan. Weapons flow from the Dominion of Pakistan to the rebels, and on 22nd October, Pakistani soldiers join the fray and invade the independent Princely State of Kashmir and Jammu. Having no military fit to hold up against the Pakistani armed forces, Hari Singh once more reaches out to the Dominion of India, this time for military aid. The answer has three terms:
On 26th October, Hari Singh signed the Instrument of Accession for the Dominion of India. On 27th the Dominion of India accepted it, making the Princely State of Kashmir and Jammu a part of the Dominion of India. It acceded to the Dominion of India by a Bilateral contract and brought the Dominions of India and Pakistan into war. In the Ceasefire of 1948/49, some areas of Kashmir and Jammu were occupied by the Dominion of Pakistan. In 1971 the ceasefire line was declared a line of control, but to this day, no land was formally ceased, so technically it is still occupied to this day and the annexation is only de facto and not de jure.
As I am a historian, I draw my line here.
Accession is defined by the UN as:
"Accession" is the act whereby a state accepts the offer or the opportunity to become a party to a treaty already negotiated and signed by other states. It has the same legal effect as ratification. Accession usually occurs after the treaty has entered into force. 1
Hari Singh did have his Princely State join the Dominion of India, which itself was a technically a transitional body formed by the coalition of close to 700 Princely States. It was not yet one state of India but would in 1950 become the Republic of India.