The Collaborative International Dictionary of English v.0.48:

Buddha \Bud"dha\, n. [Skr. buddha wise, sage, 'the enlightened'
   fr. budh to know.]
   1. The title of an incarnation of self-abnegation, virtue,
      and wisdom.
      [1913 Webster]

   2. The title of Siddhartha or Gautama, a deified religious
      teacher of the Buddhists and the founder of Buddhism;
      called also Gautama Siddartha or Sakya Sinha (or
      Muni). From three newly discovered inscriptions of the
      emperor Asoka it follows that the 37th year of his reign
      was reckoned as the 257th from the death of Buddha. Hence
      it is inferred that Buddha died between 482 and 472 B. C.
      It being agreed that he lived to be eighty, he was born
      between 562 and 552 B. C. The Buddhist narratives of his
      life are overgrown with legend and myth. Senart seeks to
      trace in them the history of the sun-hero. Oldenberg finds
      in the most ancient traditions -- those of Ceylon -- at
      least definite historical outlines. Siddhartha, as Buddha
      was called before entering upon his great mission, was
      born in the country and tribe of the Sakhyas, at the foot
      of the Nepalese Himalayas. His father, Suddhodana, was
      rather a great and wealthy landowner than a king. He
      passed his youth in opulence at Kapila-vastu, the Sakhya
      capital. He was married and had a son Rahula, who became a
      member of his order. At the age of twenty-nine he left
      parents, wife, and only son for the spiritual struggle of
      a recluse. After seven years he believed himself possessed
      of perfect truth, and assumed the title of Buddha, 'the
      enlightened.' He is represented as having received a
      sudden illumination as he sat under the Bo-tree, or ' tree
      of knowledge,' at Bodhgaya or Buddha-Gaya. For
      twenty-eight or, as later narratives give it, forty-nine
      days he was variously tempted by Mara. One of his doubts
      was whether to keep for himself the knowledge won, or to
      share it. Love triumphed, and he began to preach, at first
      at Benares. For forty-four years he preached in the region
      of Benares and Behar. Primitive Buddhism is only to be
      gathered by inference from the literature of a later time.
      Buddha did not array himself against the old religion. The
      doctrines were rather the outgrowth of those of certain
      Brahmanical schools. His especial concern was salvation
      from sorrow, and so from existence. There are "four noble
      truths": (1) existence is suffering; (2) the cause of pain
      is desire, (3) cessation of pain is possible through the
      suppression of desire; (4) the way to this is the
      knowledge and observance of the "good law " of Buddha. The
      end is Nirvana, the cessation of existence. Buddhism was
      preached in the vulgar tongue, and had a popular
      literature and an elaborately organized monastic and
      missionary system. It made its way into Afghanistan,
      Bactriana., Tibet, and China. It passed away in India not
      from Brahman persecution, but rather from internal causes,
      such as its too abstract nature, too morbid view of life,
      relaxed discipline, and overgrowth of monasticism, and
      also because Shivaism and Vishnuism employed many of its
      own weapons more effectively. The system has been
      variously modified in dogma and rites in the many
      countries to which it has spread. It is supposed to number
      about 850,000,000 of adherents, who are principally in
      Ceylon, Tibet, China, and Japan.
      [Century Dict. 1906.]

WordNet (r) 3.0 (2006):

Buddha
    n 1: founder of Buddhism; worshipped as a god (c 563-483 BC)
         [syn: Buddha, Siddhartha, Gautama, Gautama
         Siddhartha, Gautama Buddha]
    2: one who has achieved a state of perfect enlightenment

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "Buddha":
   Confucius, Gandhi, Jagannath, Juggernaut, Kurma, Mahavira, Matsya,
   Mentor, Muhammad, Narsinh, Nestor, Parshuram, Plato, Rama,
   Socrates, Solomon, Vaman, Varah, Vardhamana, Zoroaster,
   bodhisattva, the Blessed One, the Lord Buddha, the Teacher