bas-relief:  or low-relief; a kind of sculpture that is cut only a little way into a stone background. It is like a picture in stone.

subtractive sculpture:  the carving or chiseling process used for wood or stone sculptures. The artist begins with a piece of wood or stone and cuts away what is not needed until the finished sculpture emerges.

contour line:  the outside edge of an object and the lines that are inside the main shape. People usually draw the outside contours of objects first, and then draw the inside contours.

(Source:  Definitions by Maryan Infield, Middle School Teacher)