This is a tough question. Gender seems to be an uber-adjective we use to categorize people. When we learn someone's gender, we to assign to them a group of stereotypes. Gender = Sex + Orientation + Cultural Affiliation. It's so broad it's hard to define, but we seem to understand it so well.
Gender doesn't need a definition for me. It's just a label we need to use to simplify things. Gender could be anything from the way you see yourself to the way you put your shoes on it doesn't make a difference it all ends up being you and coming back to how you feel.
My Gender is simple. My name is Evan and I'm a chill little dude and I may not be like that to someone else but that's just how I see it =]
I guess the way you can define gender is like this: Gender's definition = Definition of yourself