(countable, uncountable) The foam made by rapidly stirring soap and water.

(countable, uncountable) Foam from profuse sweating, as of a horse.

(countable) A state of agitation.

(transitive) To cover with lather.

(transitive) To beat or whip.

(intransitive) To form lather or froth, as a horse does when profusely sweating.