Something given in return for a item of equivalent value – like tit for tat.
A Latin term meaning ‘something for something’ or ‘this for that’. The idea is more commonly expressed in English as ‘one good turn deserves another’.

It has been in use in the English speaking world since the 16th century. An early example of it in print is found in Erasmus’ Lytle Treatise Maner & Forme of Confession, 1535:

Poticaries and phisions do more greuously offende, than do these persones now rehersed, which haue a prouerbe amonge them, quid pro quo, one thynge for another.