The Collaborative International Dictionary of English v.0.48:

Caulk \Caulk\, v. t.
   1. See Calk, v..
      [1913 Webster]

   2. to fill in the seams or cracks of, with a waterproof
      material such as caulk.
      [PJC]

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]

WordNet (r) 3.0 (2006):

caulk
    n 1: a waterproof filler and sealant that is used in building
         and repair to make watertight [syn: caulk, caulking]
    v 1: seal with caulking; "caulk the window" [syn: caulk,
         calk]