Foam  —n. 1 mass of small bubbles formed on or in liquid by agitation, fermentation, etc. 2 froth of saliva or sweat. 3 substance resembling these, e.g. Spongy rubber or plastic. —v. Emit or run with foam; froth.  foam at the mouth be very angry.  foamy adj. (-ier, -iest). [old english]