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]