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.