So, according to your link:

Definition:

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.

“It depends on what the meaning of the word ‘is’ is.”