Source : Webster's Revised Unabridged Dictionary (1913)

Factorial \Fac*to"ri*al\, a.
   1. Of or pertaining to a factory. --Buchanan.

   2. (Math.) Related to factorials.

Factorial \Fac*to"ri*al\, n. (Math.)
      (a) pl. A name given to the factors of a continued product
          when the former are derivable from one and the same
          function F(x) by successively imparting a constant
          increment or decrement h to the independent variable.
          Thus the product F(x).F(x + h).F(x + 2h) . . . F[x +
          (n-1)h] is called a factorial term, and its several
          factors take the name of factorials. --Brande & C.
      (b) The product of the consecutive numbers from unity up
          to any given number.

Source : WordNet®

factorial
     adj : of or relating to factorials
     n : the product of all the integers up to and including a given
         integer; "1, 2, 6, 24, and 120 are factorials"