1.nounA distinguishing feature of a person or thing.
2.nounThe integer part of a logarithm.
3.nounThe distinguishing features of a navigational light on a lighthouse etc by which it can be identified (colour, pattern of flashes etc.).
4.nounFor a given field or ring, a natural number that is either the smallest positive number n such that n instances of the multiplicative identity (1) summed together yield the additive identity (0) or, if no such number exists, the number 0.
"The characteristic of a field, if non-zero, must be a prime number."