The Collaborative International Dictionary of English v.0.48:

Buddhism \Bud"dhism\, n.
   The religion based upon the doctrine originally taught by the
   Hindu sage Gautama Siddartha, surnamed Buddha, "the awakened
   or enlightened," in the sixth century b. c., and adopted as a
   religion by the greater part of the inhabitants of Central
   and Eastern Asia and the Indian Islands. Buddha's teaching is
   believed to have been atheistic; yet it was characterized by
   elevated humanity and morality. It presents release from
   existence (a beatific enfranchisement, Nirv[^a]na) as the
   greatest good. Buddhists believe in transmigration of souls
   through all phases and forms of life. Their number was
   estimated in 1881 at 470,000,000.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Buddhism
    n 1: a religion represented by the many groups (especially in
         Asia) that profess various forms of the Buddhist doctrine
         and that venerate Buddha
    2: the teaching of Buddha that life is permeated with suffering
       caused by desire, that suffering ceases when desire ceases,
       and that enlightenment obtained through right conduct and
       wisdom and meditation releases one from desire and suffering
       and rebirth