The Collaborative International Dictionary of English v.0.48:

passe-partout \passe`-par`tout"\, Passe partout \Passe"
par`tout"\(p[aum]s` p[aum]r`t[=oo]"), n. [F., from passer to
   pass + partout everywhere.]
   1. That by which one can pass anywhere; a safe-conduct.
      [Obs.] --Dryden.
      [1913 Webster]

   2. A master key; a latchkey. [WordNet sense 1]
      [1913 Webster]

   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.
      [1913 Webster]

   4. A strip of adhesive paper or gummed tape used to mount a
      picture between a piece of glass and a backing. [WordNet
      sense 2]
      [WordNet 1.5]

   5. The method of mounting a picture between a piece of glass
      and a backing, using a passe-partout[4].
      [PJC]

WordNet (r) 3.0 (2006):

passe-partout
    n 1: key that secures entrance everywhere [syn: passkey,
         passe-partout, master key, master]
    2: a mounting for a picture using gummed tape