word-forming element meaning "level, flat, plane," from Latin plani-, from planus "flat, level" (from PIE root *pele- (2) "flat; to spread").

before vowels plagi-, word-forming element meaning "slanting, oblique," from Greek plagios "oblique, slanting," from plagos "side," from PIE *plag- "flat, spread," variant form of root *plak- (1) "to be flat."

scientific word-forming element meaning "smooth," from Greek leios "smooth, level, flat; plain, unembroidered; beardless." E.g. leiotrichy, in ethnology, of races, "condition of having straight, lank hair" (1924).

word-forming element used scientifically, "of or pertaining to lakes and fresh water," from Greek limne "pool of standing water, tidal pool, marsh, lake," a word of uncertain origin; the most likely guess is that it is related to Latin limus "mud," from PIE root *(s)lei- "slime" (see slime (n.)), via the notion of "moistness, standing water" [Beekes].