1. (adj) A behavior utilizing privilege & power based on their gender to in some way oppress the non-dominant gender
. Because "men" and "women" are the two most-acknowleged genders, and men are currently the dominant group, sexism is currently used against women. Were the societal roles reversed, women could use sexism against men, but currently they cannot. They can, however "discriminate unfairly" but without the privilege and power to back it up, this behavior cannot be termed sexism
2. (n) A person exemplifying sexist behavior.