What is "syn-"

Syn- \Syn-\ [Gr. sy`n with.] A prefix meaning with, along with, together, at the same time. Syn- becomes sym- before p, b, and m, and syl- before l.

word-forming element meaning "together with, jointly; alike; at the same time," also sometimes completive or intensive, from Greek syn (prep.) "with, together with, along with, in the company of," from PIE *ksun- "with" (cognates: Russian so- "with, together," from Old Russian su(n)-). Assimilated to -l-, reduced to sy- before -s- and -z-, and altered to sym- before -b-, -m- and -p-. Since 1970s also with a sense of "synthetic."

pre. 1 identical 2 with, together 3 concomitant