it becomes, by assimilation, syl- before l; sym- before b, m, or p; and sys- before s or in words in which it reflects original combining with a word containing an initial
aspirate, or (h) sound
with, together, at the same time [synesthesia, syncarpous]
Webster’s New World College Dictionary, 4th Edition. Copyright © 2010 by
Houghton Mifflin Harcourt. All rights reserved.
Word origin of 'syn-'Gr < syn, with, earlier xyn