1. One who practices discriminatory behavoir based on gender. The individual may be of either sex, and may discriminate against their own sex, as well as the opposite sex.
1. Sally hates me, because she is a femenist, and I am a man, which makes her a dirty sexist.
Usually, stereotypingbased on gender. (And yes, you can be sexist to all genders, from man, woman, to non-binary.) Not just women, though usually women.