(1)a mass of small bubbles formed in or on a liquid; "the beer had a thick head of foam"
(2)液体中あるいは液体上にできた小さな泡の塊
(3)a hollow globule of gas (e.g., air or carbon dioxide)
(4)（たとえば空気や二酸化炭素の）中空の気体の小球