The Collaborative International Dictionary of English v.0.48:

Jar \Jar\ (j[aum]r), n. [F. jarre, Sp. jarra, from Ar. jarrah
   ewer; cf. Pers. jarrah.]
   1. A deep, broad-mouthed vessel of earthenware or glass, for
      holding fruit, preserves, etc., or for ornamental
      purposes; as, a jar of honey; a rose jar. --Dryden.
      [1913 Webster]

   2. The measure of what is contained in a jar; as, a jar of
      oil; a jar of preserves.
      [1913 Webster]

   Bell jar, Leyden jar. See in the Vocabulary.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Bell jar \Bell" jar`\ (Phys.)
   A glass vessel, varying in size, open at the bottom and
   closed at the top like a bell, and having a knob or handle at
   the top for lifting it. It is used for a great variety of
   purposes; as, with the air pump, and for holding gases, also
   for keeping the dust from articles exposed to view.
   [1913 Webster]

WordNet (r) 3.0 (2006):

bell jar
    n 1: a bell-shaped glass cover used to protect and display
         delicate objects or to cover scientific apparatus or to
         contain gases [syn: bell jar, bell glass]