The Collaborative International Dictionary of English v.0.48:
Trierarch \Tri"er*arch\, n. [L. trierarchus, Gr. ?; ? a trireme + ? a leader, a chief.] (Gr. Antiq.) (a) The commander of a trireme. (b) At Athens, one who (singly, or jointly with other citizens) had to fit out a trireme for the public service. [1913 Webster]