What you need to know about collusion, the term at the center of the Trump-Russia probe

PHOTO: President Donald J. Trump attends a meeting on tax policy with business leaders in the Roosevelt Room at the White House, OCT. 31, 2017.PlayMichael Reynolds/EPA
WATCH Trump's attorney reacts to Russia investigation charges

Part of the debate surrounding what did or didn't happen behind the scenes in the Trump presidential campaign stems from the vague nature of the word at the heart of the investigation: collusion.

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President Donald Trump regularly denies that there was any collusion in connection to the campaign, and the word is back at the forefront of the national conversation after he weighed in with his own assessment of the word.

"Where's the Collusion? They made up a phony crime called Collusion, and when there was no Collusion they say there was Obstruction (of a phony crime that never existed). If you FIGHT BACK or say anything bad about the Rigged Witch Hunt, they scream Obstruction!" he wrote on Twitter on Aug. 20.

Trump isn't wrong: While collusion has become the buzzword for the investigation into Russian interference in the 2016 election, the word collusion is not listed as a crime.

According to the Merriam-Webster dictionary, collusion is defined as a “secret agreement or cooperation especially for an illegal or deceitful purpose; acting in collusion with the enemy.”

The reason why the dictionary definition provides the clearest understanding is because collusion is not a legal term, according to a constitutional law scholar.

Michael Gerhardt, a constitutional law professor at the University of North Carolina, told ABC News that the vague nature of the term makes it confusing, which can be used to a political advantage.

"It's not a technical word. It's actually used, in part, to actually almost confuse people," Gerhardt said. "It's either used in two different ways: to almost confuse people, because there's no federal statute or code that uses this word, and then I think the other way it's used is maybe as a catch-all, an umbrella-like term that could encompass everything."

Jay Sekulow, a lawyer for Trump, touched on the lack of legal meaning behind the word during an interview on "Good Morning America."

"Collusion, in and of itself, there's no crime of collusion," Sekulow said.

Gerhardt said that making such a point is an example "demonstrating why they use the word collusion because if you look for it in a statute, you won't find it."

By contrast, Gerhardt said that if the word "conspiracy" was used instead, that would carry greater weight and threat of legal consequences because it's defined as a very specific type of crime.

"One problem with using the word collusion," Gerhardt said, "is that it's not used in any statute and therefore it's not codified as a crime anywhere, and that's why they use it."

Editor's Note: This report has been updated from a version that ran in October 2017.