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.
The earthquake and the power outage happened at the same time.
He shouted the moment he opened the door.
同時 refers to the same exact moment or simultaneous occurrence, while 同時期 refers to a broader same period or era.
same period; same time
same time (of day)