In the theater, a short speech that the character addresses to the audience, or to himself, without the others on stage hearing it.
In Othello
O, you are well tuned now! / But I'll set down the pegs that make this music, / As honest as I am.
Iago comments on the couple's kiss only for the audience, unheard by Othello or Desdemona. In "I will loosen" he announces to the house the attack on the marriage, while on stage everyone sees only a happy reunion.