"a mixture of liquids insoluble in one another, where one is suspended in the other in the form of minute globules," 1610s, from French émulsion (16c.), from Modern Latin emulsionem (nominative emulsio), noun of action from past participle stem of emulgere "to milk out," from assimilated form of ex "out" (see ex-) + mulgere "to milk" (from PIE root *melg- "to rub off; to milk"). The fat (butter) in milk is the classic example of an emulsion, drops of one liquid dispersed throughout another. Sense in photography is by 1840.
"inner membrane in the stomach of an unweaned calf or other animal," used for making cheese, etc.; also the mass of curdled milk found in the stomach, mid-15c., probably from an unrecorded Old English *rynet, related to gerennan "cause to run together," because it makes milk run or curdle; from Proto-Germanic *rannijanan, causative of *renwanan "to run" (from PIE root *rei- "to run, flow"). Compare German rinnen "to run," gerinnen "to curdle." Hence, "anything used to curdle milk."