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.