Other forms: double standards
A double standard is a code or policy that favors one group or person over another. Double standards are unfair. If a teacher lets all the boys bring candy for lunch but not the girls, that’s a double standard.
A standard is a way of evaluating someone, and a double standard is two-faced. It’s like having a rule that applies to some people one way and another way to others. Men getting paid more for the same work than women is a double standard. Anytime some are held to a higher standard than others, there's a double standard because everyone isn't being treated fairly.