A Kibitzer is a Yiddish term for a person who offers (often unwanted) advice or commentary. This term can be applied to any activity, but it is most notably used to describe non-participant spectators in contract bridge, chess, go, and many other games.
The verb kibitz can also refer to idle chatting or side conversations.
In computer science the term is the title of a programming language released by NIST, as a sub-project of the Expect programming language, that allows two users to share one shell session, taking turns typing one after another.
There is a 1930 film called The Kibitzer which is based on the 1929 three-act comedy play by the same name.
Jane Jacobs describes a kibitzer as someone who keeps a look-out on a street, and seeing suspicious activity, intervenes to help the victim. In this way, kibitzers help keep streets safe.