Sentence:  Perhaps because of this, the convention of the <WSD>'stage whisper</WSD>' came into being. All that distinguishes it from the true whisper is that a very little amount of voice in introduced and so what you end up with is extremely breathy voice production... 
Meaning of the word marked with <WSD> tags:  A line that is performed on stage as if it were whispered, but is spoken loud enough that the audience can hear.