Used as a noun or adverbially with に to indicate that two events happen at the same time.
They arrived at the same time.
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.
Simultaneous interpretation is a very difficult job.
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)