Treating people differently because of their gender.
Sex-based discrimination against a person. The concept was coined along the lines of concepts of racism (race-based discrimination) and ageism (violation of the rights of the elderly). It was put into circulation by American feminist, political scientist Kirsten Amundsen in her book "Silent majority. Women and American democracy" (1971).
Examples
A woman is being paid less than a man for doing the same job. (or not even getting the job) (example: professional soccer player)
A man is being paid less than a woman for doing the same job. (or not even getting the job) (example: professional fashion model)
Men wearing "female" clothing or jewelry (anything pink, anything skirt or dress like,etc..) are not accepted in public while women can wear (almost) whatever they want, from black leather pants to mini skirt.
Men can boast about having many different (and younger) sexual partners, while women are called bitches for doing the same thing.
Women are accepted to kiss and hug their friends and in general showing their emotions, men are raised to hide their emotions and are called gay for doing the same.