According to the Illustrated Dictionary of Historic Architecture by C.M. Harris a corbel is
1.  In masonry, a projection or one of a series of projections, each stepped progressively farther forward with height; anchored in a wall, story, column, or chimney; used to support an overhanging member above or, if continuous, to support overhanging courses; may support an overhanging ornament of similar appearance.

2.   A projecting stone which supports a supericumbent weight.