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.-to gather foam; to froth; as, the billows foam.-to form foam, or become filled with foam; -- said of a steam boiler when the water is unduly agitated and frothy, as because of chemical action.-to cause to foam; as,to foam the goblet; also (with out), to throw out with rage or violence, as foam.