Caulk is the sealant. It comes in a tube and is used commonly by plumbers and carpenters trying to seal up cracks where air or water might push through. When a plumber caulks a tub, he's applying caulk to its seams to make it watertight.

Caulk can be used to describe both the process of applying caulk and the sealant itself. The word caulk comes from the Old Northern French cauquer, meaning "to press down." After you lay the caulk down over the seam you press it down by either running your finger over it or using a specific tool to force the caulk to settle into the hole you're trying to cover.

Definitions of caulk

n a waterproof filler and sealant that is used in building and repair to make watertight

Type of:
sealant, sealer
a kind of sealing material that is used to form a hard coating on a porous surface (as a coat of paint or varnish used to size a surface)

v seal with caulking

caulk the window”
Type of:
seal, seal off
make tight; secure against leakage