When an actor turns to speak directly to the audience, unheard by the other characters, it is known as an "aside"?

This statement is true.

An aside is a term which refers to a line or a monologue that one of the characters in the play is addressing to the audience directly. It is a form of 'breaking the fourth wall' because the character knows they are being watched by an audience and is speaking to them explicitly. That statement is known as an aside in drama terminology.