A character in a story or play who is there not for his/her character development or role in the story, but solely for the purpose of providing exposition to the audience.
Any time two minor characters in a Shakespearean play have a conversation not about themselves, but about others. Examples are found in Macbeth (2.4, 3.6), where the exposition monkey is the Old Man (and sometimes Ross); Merchant of Venice (2.8, first half of 3.1), where the exposition monkey is Solanio, Salarino, and Salerio. Some television characters serve this function as well as being regular characters. For example, Donna Moss on The West Wing.