Quid pro quo

"Quid pro quo" is a Latin phrase. It means roughly "this for that." In other words, the phrase describes the context when something is given in return for something else.
The phrase means the same thing as the English saying "a tit for a tat", or trading something for something else. It can also mean the same thing as "an eye for an eye".
Lawyers sometimes use this saying to mean "an even trade".