noun
Used as a noun or adverbially with に to indicate that two events happen at the same time.
彼らは同時に到着した。
They arrived at the same time.
電話とメールが同時に来た。
A phone call and an email came at the same time.
noun-modifying (irregular)
simultaneous; concurrent; synchronous
Prenominal use modifying a noun to describe events, actions, or processes that occur at the same time.
同時通訳はとても難しい仕事だ。
Simultaneous interpretation is a very difficult job.
二つの同時進行のプロジェクトを管理している。
I'm managing two concurrent projects.