word-forming element usually meaning "with, together," from Latin com, archaic form of classical Latin cum "together, together with, in combination," from PIE *kom- "beside, near, by, with" (cf. Old English ge-, German ge-). The prefix in Latin sometimes was used as an intensive.
Before vowels and aspirates, reduced to co-; before -g-, assimilated to cog- or con-; before -l-, assimilated to col-; before -r-, assimilated to cor-; before -c-, -d-, -j-, -n-, -q-, -s-, -t-, -v- assimilated to con-.
com- or col- or con-
Together; with; joint; jointly: commensalism.