The term “obstruction of justice” has been swirling inside Washington D.C. and across cable television ever since President Donald Trump fired FBI Director James Comey last week. The talk only intensified after the New York Times cited a memo from Comey claiming Trump had asked him to shut down an investigation into former National Security Adviser Michael Flynn following his resignation.

“Lots of chatter from Ds and Rs about the exact definition of “obstruction of justice,” Senator Chris Murphy tweeted Tuesday with a link to the Times report.

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But the key to proving obstruction of justice, explains Robert Weisberg, a law professor at Stanford, is that the intervention has to be propelled by corrupt motives.

“If it’s a threat, that makes it a crime. If it’s not a threat — but a request — it could still be a crime if the threat is motivated by a corrupt purpose,” Weisberg said.