The piece of a guitar or other stringed instrument with one or more holes in it to let the sound out.
A thin wooden or skin surface which vibrates in sympathy with the strings and conspires with the rest of the body to amplify the sound.
The material, thickness and shape of the soundboard all contribute to the quality and volume of the sound produced. Wooden soundboards are most often Spruce but cheap instruments tend to use plywood. This can normally be easily determined by looking at the cross section in the sound hole which is usually left unbound. Cedar and Redwood are also used as an alternative to Spruce.
Skin soundboards are used in banjos, where they are called heads, deriving from the drum making tradition. Originally vellum (usually calf skin) they now tend to be synthetic (mylar plastic) and pre-formed with an aluminium ring. Synthetic heads are every bit as good, retain tension longer, do not suffer from humidity and are much more pleasant to fit! Only authentic restorations really use vellum these days.