n. A sharp burst of thunder; a sudden report of a discharge of atmospheric electricity. “Thunderclaps that make them quake.” Spenser. [ 1913 Webster ]
When suddenly the thunderclap was heard. Dryden. [ 1913 Webster ]
n. (Geol.) A stratum of clay lying beneath a coal bed, often containing the roots of coal plants, especially the Stigmaria. [ 1913 Webster ]