What is "passe partout"

Passe partout \Passe" par`tout"\, n. [F., from passer to pass + partout everywhere.]

1. That by which one can pass anywhere; a safe-conduct. [Obs.]
   --Dryden.

2. A master key; a latchkey.

3. A light picture frame or mat of cardboard, wood, or the like, usually put between the picture and the glass, and sometimes serving for several pictures.