zugleich
simultaneously
At exactly the same time; at once.
Die beiden Läufer überquerten die Ziellinie zugleich.
The two runners crossed the finish line simultaneously.

simultaneously / also
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simultaneously
At exactly the same time; at once.
Die beiden Läufer überquerten die Ziellinie zugleich.
The two runners crossed the finish line simultaneously.

The word is made up of 'zu' and 'gleich'. It expresses that two things come together ('zu') at the same moment or in the same state ('gleich').
Er ist mein Kollege und zugleich mein bester Freund.
He is my colleague and also my best friend.
As an adverb, 'zugleich' (simultaneously/also) can stand at the beginning of a sentence and then causes inversion (verb in position 2). But it can also stand in the middle of the sentence, usually after the conjugated verb.
Zugleich wurde ihm klar, dass er einen Fehler gemacht hatte.
At the same time, he realized that he had made a mistake.
The meaning depends on the context. If 'zugleich' (simultaneously) refers to two actions, it means 'at the same time'. If it connects two qualities or roles of a person or thing, it means 'in addition' or 'also'.
Er kam an und ging zugleich wieder.
He arrived and left again at the same time.
also / even
'auch' and 'zugleich' can both express an additional quality or function. 'auch' adds something in a general way. 'zugleich' often means that two things happen at the same time, or that something also has another role at the same time. That is why 'zugleich' is more precise when time or simultaneity is important. Learners confuse the words because 'zugleich' can sometimes feel similar to 'auch'. Remember: 'auch' simply means 'also'; 'zugleich' often means 'also and at the same time'.

Die Eltern planen die Reise und buchen zugleich die Unterkunft.
The parents plan the trip and book the accommodation at the same time.
Zu Hause hat meine Nachbarin auch meinen Freund begrüßt.
At home, my neighbor also greeted my friend.