double factorial (plural double factorials)
- (mathematics, combinatorics) For an odd integer, the result of multiplying all the odd integers from 1 to the given number; or for an even integer, the result of multiplying all the even integers from 2 to the given number; symbolised by a double exclamation mark (!!). For example, 9!! = 1 * 3 * 5 * 7 * 9 = 945.
- MathWorld, Double Factorial