"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.
Proto-Indo-European root meaning "to rub off," also "to stroke; to milk," in reference to the hand motion involved in milking an animal. Compare *g(a)lag-.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit marjati "wipes off;" Greek amelgein, Latin mulgere, Old Church Slavonic mlesti, Lithuanian melžti "to milk;" Old Irish melg "milk."