1) In general
Latin for "something for something." An exchange of acts or things of approximately equal value.
2) In employment law
Sexual harassment in which a boss conveys to an employee that he or she will base an employment decision, e.g. whether to hire, promote, or fire that employee, on the employee's satisfaction of a sexual demand. For example, it is quid pro quo sexual harassment for a boss to offer a raise in exchange for sex.