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.
See, e.g. Citizens United v. Federal Election Comm'n, 130 S.Ct. 876 (2009) (generally) and Burlington Industries, Inc. v. Ellerth, 524 U.S. 742 (1998) (regarding quid pro quo sexual harassment).