1) Discrimination based on one's gender.
2) Something that is rampant on this website. Many assholes on the site confuse feminists (who strive for women's rights) with radical feminists (who just hate all men).
3) Something women have had to suffer from since the dawn of mankind. Men can suffer mild forms of it too, but it is more common to women.
Look up "feminism" "females" or "women's rights" at urbandictionary.com and you will find definitions laced with sexism.