zeitgleich
simultaneous
Describes that two or more events or actions take place at exactly the same time.
Die beiden Läufer überquerten zeitgleich die Ziellinie.
The two runners crossed the finish line simultaneously.

simultaneous
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simultaneous
Describes that two or more events or actions take place at exactly the same time.
Die beiden Läufer überquerten zeitgleich die Ziellinie.
The two runners crossed the finish line simultaneously.

The word is made up of 'Zeit' (time) and 'gleich' (same/equal). So it literally means “at the same time,” which makes it easy to understand.
Die Ampeln schalteten zeitgleich auf Grün.
The traffic lights turned green simultaneously.
'Zeitgleich' (simultaneous/simultaneously) is mostly used like an adverb. It can stand at the beginning of a sentence, which moves the verb to position 2, or in the middle of the sentence.
Zeitgleich mit dem Film startete auch eine neue Serie.
At the same time as the film, a new series also started.
'Zeitgleich' (simultaneous/simultaneously) is often combined with verbs that describe an event. Typical verbs are 'stattfinden' (to take place), 'beginnen' (to begin), 'enden' (to end), or 'ankommen' (to arrive).
Die Explosionen fanden an zwei Orten fast zeitgleich statt.
The explosions took place in two locations almost simultaneously.
simultaneously / at the same time
'gleichzeitig' and 'zeitgleich' both describe that two or more things happen at the same time. The difference is mainly in usage. 'gleichzeitig' is an adverb and fits very naturally in everyday sentences: you do two things 'gleichzeitig' (at the same time). 'zeitgleich' is an adjective and often describes events or processes, for example 'zeitgleiche Termine' (appointments at the same time) or 'zeitgleich stattfindende Kurse' (courses taking place at the same time). They can be confused because the meaning is very similar. But pay attention to the grammar: 'gleichzeitig' often goes with a verb, while 'zeitgleich' more often describes a noun or is used with 'sein' (to be).

Die beiden Besprechungen sind zeitgleich, deshalb kann ich nur an einer teilnehmen.
The two meetings are at the same time, so I can only attend one.
Im Büro klingeln heute ständig mehrere Telefone gleichzeitig.
At the office today, several phones keep ringing at the same time.