A vampire number is a composite number with an even number of digits that can be written as the product of two fangs — two factors, each with half as many digits, whose digits taken together are a rearrangement of the original number's digits. The classic first example is 1260 = 21 × 60.
The rules rule out trivial cases: the two fangs must each have exactly half the digits, and they can't both end in zero (otherwise 126000 = 210 × 600 and friends would flood the list). The first vampire numbers are 1260, 1395, 1435, 1530, 1827, 2187, 6880, 102510, 104260.
Vampire numbers were coined by Clifford Pickover in 1994 and are a staple of recreational number theory; some have multiple distinct fang pairs, making them "double" or "triple" vampires.