Quid pro quo
Etymology
A Latin phrase meaning literally "something for something" or "what for what" (quid, "what, something," repeated around pro, "for, in place of"). It originally referred to substituting one thing for another, including one medicine for another by mistake. It settled into English as the name for a reciprocal exchange, often one with a whiff of impropriety.
noun phrase · /kwid-proh-KWOH/
An exchange in which one thing is given or done in return for another.
What it names
The favor-for-favor arrangement at the heart of many bribery and harassment claims — "do this for me and I’ll do that for you."
Example
Prosecutors argued the donation was a quid pro quo for the senator’s vote.
Related idea
A quid pro quo is the trade itself; caveat emptor is a warning about a different risk — that a lawful trade may still leave you with a bad bargain.