The Collaborative International Dictionary of English v.0.48:

Factum \Fac"tum\ (f[a^]k"t[u^]m), n.; pl. Facta. [L. See
   Fact.]
   1. (Law) A man's own act and deed; particularly:
      (a) (Civil Law) Anything stated and made certain.
      (b) (Testamentary Law) The due execution of a will,
          including everything necessary to its validity.
          [1913 Webster]

   2. (Mach.) The product. See Facient, 2.
      [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

FACTUM. A deed. a man's own act and deed.
     2. When a man denies by his plea that he made a deed on which he is
sued, be pleads non est factum. (q.v.) Vide Deed; Fait.



Bouvier's Law Dictionary, Revised 6th Ed (1856):

FACTUM, French law. A memoir which contains summarily the fact on which a
contest has happened, the means on which a party founds his pretensions,
with the refutation of the means of the adverse party. Vide Brief.