Sentence:  All are based on the concept of <WSD>"quid pro quo</WSD>", or “this for that” – the exchange of one thing for another. ... In McDonnell, the government alleged a <WSD>"quid pro quo</WSD>" sale of “official action” – payments (the "quid") made to McDonnell, the former governor of Virginia, in return for a promise or undertaking by McDonnell to perform an official action (the "quo"). The Court's decision focused on the "quo" side of the equation – a side that does not often receive much scrutiny – analyzing the contours of whether the actions taken by McDonnell constituted illegal “official actions.” 
Meaning of the word marked with <WSD> tags:  A usually non-monetary exchange transaction, or series or process of exchange transactions.