Annexation,[1] in international law, is the forcible acquisition and assertion of legal title over one state’s territory by another state, usually following military occupation of the territory.
Opinion of Internation Law:
In current international law, it is generally held to be an illegal act.
International law says its illegal. The word doesn’t define it being inherently illegal.
The international law also says a lot of other things are illegal, but those acts are not inherently illegal, because the legality isn’t inherent to the word’s definition.
https://en.wikipedia.org/wiki/Annexation
So, according to your link:
Definition:
Opinion of Internation Law:
International law says its illegal. The word doesn’t define it being inherently illegal.
The international law also says a lot of other things are illegal, but those acts are not inherently illegal, because the legality isn’t inherent to the word’s definition.