
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.

dubbed
Refers to the dubbing of films, where the actors' lip movements match the translated audio track.
Der Film ist so gut synchron, dass man die Originalsprache nicht vermisst.
The film is so well dubbed that you don't miss the original language.
'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” can say that two processes take place at the same time. “Synchron” additionally emphasizes that the processes are precisely coordinated with each other, while “gleichzeitig” does not necessarily express this coordination. Choose “synchron” when there is a shared rhythm or precise coordination, and “gleichzeitig” when things merely happen at the same time.

Die beiden Tänzer sprangen synchron in die Luft.
The two dancers jumped into the air in sync.
Die beiden Tänzer sprangen gleichzeitig in die Luft.
The two dancers jumped into the air at the same time.