confluence (n.)

word-forming element meaning "together, with," sometimes merely intensive; it is the form of com- used in Latin before consonants except -b-, -p-, -l-, -m-, or -r-. In native English formations (such as costar), co- tends to be used where Latin would use con-.

1580s, "flowing freely" (of water), also, of speakers, "able and nimble in the use of words," from Latin fluentem (nominative fluens) "lax, relaxed," figuratively "flowing, fluent," present participle of fluere "to flow, stream, run, melt," from extended form of PIE *bhleu- "to swell, well up, overflow" (source also of Latin flumen "river;" Greek phluein "to boil over, bubble up," phlein "to abound"), an extended form of root *bhel- (2) "to blow, swell." Used interchangeably with fluid (adj.) in 17c. in the sense "changeable, not rigid." Related: Fluently.