Sexism is discrimination based on the gender of a person -- looking down on people because they are male or female. Sexism may also include stereotyped roles for men and women. International laws, such as Convention on the Elimination of All Forms of Discrimination against Women or Yogyakarta Principles demand to abolish any kind of sexism. And it might affect the male or female attending a party, getting a job that means being hired, joining a game, promotion, being selected and so on.
Although Sexism is not illegal in many states and countries, it is frowned upon.