Sentence:  The <WSD>characteristic</WSD> of a field, if non-zero, must be a prime number. 
Meaning of the word marked with <WSD> tags:  For 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.