It's THE Buddha, not Buddha, because it was his title. It's kind of like THE Pope or THE Dalai Lama, except that there was only one Buddha. His name, incidentally, was Siddharta.

Picky, picky.

He was Prince Siddhartha, but when he acheived enlightenment people called him Buddha, "The Awakened One". Most forms of buddhism recognize other Buddhas, which is why Padwan is right: it's correct to call him "The" Buddha, to distinguish him from others. Or, Shakamuni Buddha (in the Japanese tradition or Guatama (sp?) Buddha in the Indian tradition. No clue what the leading terms mean, to tell the truth.

Ok, so as you can see now I've retitled this topic to include "The".

Well, if you include the Bodisatvas...