Foam (fōm), n. [OE. fam, fom, AS. fām; akin to OHG. & G. feim.] The white substance, consisting of an aggregation of bubbles, which is formed on the surface of liquids, or in the mouth of an animal, by violent agitation or fermentation; froth; spume; scum; as, the foam of the sea.
Foam cock, in steam boilers, a cock at the water level, to blow off impurities.