Within hours after the American public found out that his most senior campaign official was under an indictment that described him as a secret agent of Russian interests, President Trump declared on Twitter: “There is NO COLLUSION!” A forceful statement if there ever was one. But what exactly was he denying so categorically? We have no idea.

For one reason or another, “collusion” has become the term of choice for discussing what the Trump campaign may or may not have done with Russians. Those in the Trump camp use it regularly: “I did not collude, nor know of anyone else in the campaign who colluded, with any foreign government,” Jared Kushner told Congress this summer. “I did not collude with any foreign government,” Donald Trump Jr. said. “I deeply resent any allegation that I would collude with the oppressive Russian state,” the Republican strategist Roger Stone harrumphed.

It isn’t just those in the Trump camp, though, who have settled on using this word. Among the first to refer to collusion was John Podesta, Hillary Clinton’s campaign chairman, who raised the specter on “Meet the Press”: “I would argue that there’s very, it’s very much unknown whether there was collusion.” In that same week, Senator Harry Reid, Democrat of Nevada, said of Trump campaign connections to WikiLeaks, “So there is collusion there, clearly.” The term has been a touchstone ever since.

Mr. Trump and his inner circle have benefited enormously from this coalescing around the word “collusion” — a term with a legalistic feel but with close to “no legal meaning whatsoever” said Renato Mariotti, a former federal prosecutor and now a defense lawyer who has written a dissection of every public statement that a Trump associate has made to congressional investigators. If we care about the law — and about holding public figures accountable for their false denials — the impassioned disavowals of collusion by members of the Trump circle mean nothing. Donald Trump Jr.’s utterances to Congress, for example, were “not denying that he committed a crime,” Mr. Mariotti said. “Whether his denial is broader or more narrow than that depends on what exactly is meant by ‘collude’ in this statement — which we don’t know.”