IPA: //ˌproʊtəˈmɑːrtər//
KK: /proʊtəˈmɑrtər/
A person who is the first to die for a particular cause, especially in a religious context, often referring to Saint Stephen as the first Christian martyr.
Saint Stephen is recognized as the protomartyr of Christianity.
Protomartyr is formed from "proto-" (meaning first) and "martyr" (from Greek "martus", meaning witness). The term refers to the first person to be martyred for their faith, thus a 'first witness' in the context of martyrdom.
Think of 'proto-' meaning first, and 'martyr' meaning witness. This helps you remember that a protomartyr is the first witness to suffer for their beliefs.
No commonly confused words.