A soundboard (or sounding board) is the surface of a string instrument that the strings vibrate against, usually via some sort of bridge. Depending on the instrument, it is also called a top, top plate, resonator, table, sound-table, or belly. It is usually made of a softwood, often spruce.
Pianos, guitars, banjos, and many other stringed instruments incorporate soundboards. The resonant properties of the soundboard and the interior of the instrument greatly increase the loudness of the vibrating strings. "The sound board is probably the most important element of a guitar in terms of its influence on the quality of the instrument's tone [timbre]."