The Collaborative International Dictionary of English v.0.48:

Sealing wax \Seal"ing wax`\
   A compound of the resinous materials, pigments, etc., used as
   a material for seals, as for letters, documents, etc.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sealing wax
    n 1: fastener consisting of a resinous composition that is
         plastic when warm; used for sealing documents and parcels
         and letters [syn: sealing wax, seal]