Merriam-Webster's Collegiate® Dictionary, 11th Edition
SNOWBALL
I. noun
Date: 15th century
1. a round mass of snow pressed or rolled together, snow cone , any of several cultivated shrubby viburnums (genus Viburnum ) with clusters of white sterile flowers,
II. verb
Date: 1843
intransitive verb to engage in throwing snowballs, to increase, accumulate, expand, or multiply at a rapidly accelerating rate, transitive verb to throw snowballs at, to cause to increase or multiply at a rapidly accelerating rate