Meaning of underpainting

un•der•paint•ing

Pronunciation: (un'dur-pān"ting), [key]
— n.
  1. the first coat of paint, esp. the initial painting on a canvas in which the major areas, tones, colors, and forms are indicated in mass.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.