The playing or singing of notes at the same pitch by different instruments or voices. (Cambridge Dictionary)
Simultaneous performance or utterance of action or speech. (Oxford Dictionaries)
If two or more people do something in unison, they do it together at the same time. (Collins Dictionary)
Identity in musical pitch. (Merriam – Webster)
How do flocking birds move in unison?
Is two singing voices occasionally being unison ”allowed”?