Dictionary.net

bä-re-l[=e]f', BASS-RELIEF, bas're-l[=e]f', n. (sculp.) figures which do not stand far out from the ground on which they are formed--also used in the Italian form BASS'O-RILIE'VO. [See BASE, low, and RELIEF.]

(Piece of) shallow carving or sculpture on background (less than half the true depth). [French]

See Basso-relievo.