The Collaborative International Dictionary of English v.0.48:

Memoir \Mem"oir\, or pl. Memoirs \Mem"oirs\, n. [F. m['e]moire,
   m., memorandum, fr. m['e]moire, f., memory, L. memoria. See
   Memory.]
   1. A memorial account; a history composed from personal
      experience and memory; an account of transactions or
      events (usually written in familiar style) as they are
      remembered by the writer. See History, 2.
      [1913 Webster]

   2. A memorial of any individual; a biography; often, a
      biography written without special regard to method and
      completeness.
      [1913 Webster]

   3. An account of something deemed noteworthy; an essay; a
      record of investigations of any subject; the journals and
      proceedings of a society.
      [1913 Webster]

WordNet (r) 3.0 (2006):

memoir
    n 1: an account of the author's personal experiences
    2: an essay on a scientific or scholarly topic

Moby Thesaurus II by Grady Ward, 1.0:

129 Moby Thesaurus words for "memoir":
   Clio, Muse of history, account, accounts, adventures, adversaria,
   aide-memoire, anecdote, annals, annotation, article, autobiography,
   biographical sketch, biography, case history, causerie, chronicle,
   chronicles, chronology, commitment to memory, confessions,
   curriculum vitae, descant, diary, discourse, discussion,
   disquisition, dissertation, docket, entry, essay, etude,
   examination, excursus, exercise of memory, experiences, exposition,
   feature, first approach, flashback, footnote, fortunes,
   hagiography, hagiology, hindsight, historiography, history, homily,
   introductory study, item, jotting, journal, learning by heart,
   legend, letters, life, life and letters, life story, looking back,
   lucubration, marginal note, marginalia, martyrology, memo, memoirs,
   memorabilia, memorandum, memorial, memorials, memories,
   memorization, memorizing, memory, minutes, monograph, morceau,
   narrative, necrology, notation, note, obituary, outline, pandect,
   paper, paragraph, photobiography, piece, preliminary study,
   profile, prolegomenon, recall, recalling, recollecting,
   recollection, reconsideration, record, records, reflection,
   register, registry, remembering, remembrance, reminder,
   reminiscence, report, reportage, research paper, resume,
   retrospect, retrospection, review, rote, rote memory, scholia,
   scholium, screed, sketch, special article, story, study, survey,
   term paper, theme, theory of history, thesis, tract, tractate,
   treatise, treatment