The Collaborative International Dictionary of English v.0.48:

Protomartyr \Pro"to*mar`tyr\, n. [LL., fr. Gr. ?; ? first + ?
   martyr: cf. F. protomartyr. See Proto-, and Martyr.]
   The first martyr; the first who suffers, or is sacrificed, in
   any cause; -- applied esp. to Stephen, the first Christian
   martyr.
   [1913 Webster]