Real Dictionary

stage whisper
countable noun: If someone says something in a stage whisper, they say it as if they are speaking privately to one person, although it is actually loud enough to be heard by other people.

stage whisper in American English
a loud whisper by an actor on the stage, heard by the audience but supposed not to be heard by other actors on stage
noun: a loud whisper on a stage, meant to be heard by the audience

stage whisper in British English
noun: a loud whisper from one actor to another on stage intended to be heard by the audience