Real Dictionary

snowball
countable noun: A snowball is a ball of snow. Children often throw snowballs at each other.
verb: If something such as a project or campaign snowballs, it rapidly increases and grows.

snowball in American English
a mass of snow packed together into a ball
to increase or accumulate rapidly and uncontrollably, like a ball of snow rolling downhill
intransitive verb: to grow or become larger, greater, more intense, etc., at an accelerating rate
noun: a ball of snow pressed or rolled together, as for throwing
transitive verb: to throw snowballs at

snowball in British English
noun: snow pressed into a ball for throwing, as in play
verb: to increase rapidly in size, importance, etc