Noun > Foam
1 : a mass of small bubbles that are formed in or on a liquid
As I poured the beer, foam bubbled up in the glass.
2 : a substance that is like a thick liquid made of many small bubbles
The fire extinguisher is filled with foam.
a can of shaving foam
3 : a soft material that is used to make many products : FOAM RUBBER — often used before another noun
a foam mattress/pillow
— foaminess noun (noncount)
— foamy adjective foamier; foamiest
a foamy glass of beer