The Collaborative International Dictionary of English v.0.48:

Diatribe \Di"a*tribe\ (?; 277), n. [L. diatriba a learned
   discussion, Gr. ?, prop., a wearing away of time, fr. ? to
   rub away, spend time; dia` through + ? to rub: cf. L. terere,
   F. trite: cf. F. diatribe.]
   A prolonged or exhaustive discussion; especially, an
   acrimonious or invective harangue; a strain of abusive or
   railing language; a philippic.
   [1913 Webster]

         The ephemeral diatribe of a faction.     --John Morley.
   [1913 Webster]

WordNet (r) 3.0 (2006):

diatribe
    n 1: thunderous verbal attack [syn: fulmination, diatribe]

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "diatribe":
   abuse, address, after-dinner speech, allocution, assailing,
   assault, attack, berating, bitter words, blackening, chalk talk,
   contumely, debate, declamation, eulogy, execration, exhortation,
   filibuster, forensic, forensic address, formal speech,
   funeral oration, harangue, hard words, hortatory address,
   inaugural, inaugural address, invective, jawing, jeremiad,
   onslaught, oration, pep talk, peroration, philippic, pitch,
   prepared speech, prepared text, public speech, rating, reading,
   recital, recitation, revilement, sales talk, salutatory,
   salutatory address, say, screed, set speech, speech,
   speechification, speeching, talk, talkathon, tirade,
   tongue-lashing, valediction, valedictory, valedictory address,
   vilification, vituperation