at the same time; simultaneously; concurrently
Used as an adverb to describe two or more actions or states occurring at the same time.
He is pursuing his job and studies at the same time.
The two started speaking simultaneously.
just as ...; the moment ...; just after ...
Used as a conjunction in the pattern ～と同時に to mean that one action happens exactly when another does, or immediately after.
The moment I opened the door, the cat jumped out.
He got married just after graduating.
at the same time (but); on the other hand; also
Used as a conjunction to introduce a contrasting or additional point, often with a nuance of 'while also' or 'but at the same time'.
This job is rewarding, but at the same time it involves a lot of stress.
He is kind. At the same time, he can also be very strict.
同時に explicitly and formally means 'at the same time', whereas 一緒に can express simultaneity more casually.