Masculinity People seem to get themselves worked up in knots over these discussions. "In my day, men were men and women were women, by gawd!" seems to be a driving sentiment from conservatives. But in every generation, there has been some blurring of the lines. There were transvestites when I was young, men grew long hair in the 1960's and when you go back even further men had long hair bound with ribbons -- sometimes powdered wigs -- and wore hose, high heel shoes, and make-up. If you could go back in time, you'd think the American founding fathers were effeminate drag queens based on their appearance.
The strictly biological answer could be about how a man is an adult human with XY chromosomes, and that is easy enough. But the more one thinks about it 'being a man' is an abstraction... it's a personal identity, a social identity, and the biological answer is only the starting point, not the end point. So, there is no definitive or all encompassing answer for what masculinity is. If I tried to take a stab at it, I'd say masculinity is a set of behaviors biological males tend to exhibit and society expects men to have, both good and bad. Since men often exhibit these behaviors and also are expected to, it forms a closed circle of selective reinforcement.