synchron
synchronous
Happening at the same time or precisely coordinated in time.
Die beiden Uhren laufen absolut synchron.
The two clocks run absolutely synchronously.

synchronous / synchronized
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synchronous
Happening at the same time or precisely coordinated in time.
Die beiden Uhren laufen absolut synchron.
The two clocks run absolutely synchronously.

'Synchron' (synchronous) belongs to a word family that has to do with simultaneity. The related verb is 'synchronisieren' (to synchronize), and the noun is 'die Synchronisation' (synchronization/dubbing).
Wir müssen unsere Uhren synchronisieren, damit wir uns zur gleichen Zeit treffen.
We need to synchronize our watches so that we meet at the same time.
The word is usually used as an adverb (how something happens) or predicatively with 'sein' (to be). It describes that two or more actions are perfectly coordinated in time.
Die Schwimmerinnen sprangen synchron ins Wasser.
The swimmers jumped into the water synchronously.
In the context of films, series, or videos, 'synchron' (dubbed/synchronized) almost always refers to the audio track. It means that the spoken text (the dubbing) matches the actors' lip movements.
Ich schaue Filme lieber im Original, auch wenn sie gut synchron sind.
I prefer watching films in the original, even if they are well dubbed.
simultaneously / at the same time
'gleichzeitig' and 'synchron' both have to do with the same time. 'gleichzeitig' simply says that two things happen at the same moment. 'synchron' often additionally means that movements or processes are precisely coordinated in time. The word is especially suitable for technology, music, dance, sports, or processes that need to fit together. Learners may confuse the words because both can mean 'at the same time'. If only the timing is important, 'gleichzeitig' fits. If something runs in exactly the same rhythm or is coordinated with something else, 'synchron' is more precise.

Die Tänzerinnen bewegen sich synchron zur Musik.
The dancers move in sync with the music.
Beim Einkaufen habe ich gleichzeitig auf die Uhr geschaut.
While shopping, I checked the time at the same time.