Tympanum

the triangular space between the horizontal and sloping cornices on the front of a pediment in classical architecture; it is often left plain, but is sometimes covered with sculpture. This name is also given to the space immediately above the opening of a doorway, etc., in mediaeval architecture, when the top of the opening is square and has an arch over it; this arrangement is not uncommon in England in Norman work, and on the Continent is to be found in each of the styles. Tympanums of this kind are occasionally perfectly plain, but are generally ornamented with carving or sculpture. In Continental work the subjects are usually arranged in tiers one above another, and often embrace a great number of figures. Also when an arch is surmounted by a gable-molding or triangular hood mould, the space included between the arch and the mould is termed the tympanum of the gable. — Parker, Gloss. of Architect. s.v.