就任 (shūnin) means “taking office” | MotiTan Japanese Dictionary

More examples

新しい社長は就任のあいさつで社員に感謝を伝えました。atara shi i sha chō wa shū nin no a i sa tsu de sha in ni kan sha o tsuta e ma shi ta

The new company president thanked the employees in his speech upon taking office.

It describes entering an official role and beginning to carry its responsibilities.

着任chakunin: The act of arriving at and taking up an assigned post.
就職shūshoku: The act of starting work in a job or profession.

役職や公職などに正式に就くこと。yaku shoku ya kō shoku na do ni sei shiki ni tsu ku ko to

就任 (shūnin) combines 就 (shū, "take up") and 任 (nin, "duty"), so it denotes taking up an official position.

In conversation, 就任 (shūnin) sounds formal because it focuses on an official position, while everyday speech more often describes ordinary job changes in simpler terms.