This article was originally published on Oct. 18. It was updated on Nov. 20.

Was there a quid pro quo involved in President Trump’s demand that Ukraine open investigations that he thought could benefit him politically, including one into former Vice President Joseph R. Biden Jr. and his son Hunter?

That question is at the heart of the impeachment inquiry against President Trump, who has denied there was any quid pro quo in his dealings with the Ukrainian president, Volodymyr Zelensky.

Which raises the question: What is a quid pro quo?

For those who do not speak Latin or who may be unacquainted with legal terminology, the basic concept behind “quid pro quo” can be summed up in a more colloquial saying in English: If you scratch my back, I’ll scratch yours.

“Quid pro quo is Latin for ‘thing for thing,’” Paul Butler, a professor at Georgetown University Law Center, said.