The Collaborative International Dictionary of English v.0.48:

Caulk \Caulk\, n.
   1. See Calk.
      [1913 Webster]

   2. a viscous semisolid material of varying composition used
      to fill in seams of objects which are exposed to water,
      such as wooden ships or bath tiles; -- called also calk
      and caulking. After applying in a semisolid form, the
      material hardens and dries to form a waterproof seal. It
      is used in the process of caulking. It is sometimes
      applied together with a rope-like cord to fill larger
      seams.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

caulking \caulk"ing\ n.
   1. the process of sealing cracks and crevices with a filler
      such as caulk[2].
      [WordNet 1.5]

   2. same as caulk[2], n..
      [PJC]

WordNet (r) 3.0 (2006):

caulking
    n 1: a waterproof filler and sealant that is used in building
         and repair to make watertight [syn: caulk, caulking]