Is it because of Mono?
Yes.
But mono as a project , or mono as a single. ?
Or both.
It probably should’ve been called MonoScript or BaseMono or WishWeThoughtOfDOTSFirst
Waiting for DOTSBehaviour…
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Yes 
Mono as in the Mono compiler. This specifier is significant because Unity used to support scripting in “Javascript” and Boo. Therefore a C# script is a Mono script*, that held a MonoBehaviour, as opposed to a Javascript script that presumably would hold an implicit JSBehaviour or something.
Behaviour as in “this is a behaviour script for the gameobject it is placed on”.
Obviously the name has been somewhat outgrown by the engine.
- To further nail the point home, .cs files are still imported as “MonoScript” objects.
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Not sure if this was ever part of the design or naming decisions but it could be pointed out that scripts added to GameObjects are generally intended to serve a single function in the Unity design specification if I’m not mistaken - thus - mono behaviour. Coincidence or intention?