WASHINGTON (NYTIMES) - The revelation that US President Donald Trump's eldest son agreed to meet a Kremlin-linked lawyer in June 2016 after being promised damaging information about Mrs Hillary Clinton has escalated discussion about whether the Trump campaign colluded with Russia's interference in the election.

An intermediary promised Mr Donald Trump Jr that a "Russian government attorney" would provide "very high level" dirt on Mrs Clinton as "part of Russia and its government's support for Mr Trump".

"If it's what you say I love it," Mr Trump Jr responded in an e-mail exchange he made public on Tuesday (July 11) after he learned The New York Times was about to publish an article about its contents.

The younger Mr Trump; his father's campaign chairman at the time, Mr Paul Manafort; and Mr Jared Kushner, the President's son-in-law and adviser, met the lawyer on June 9, 2016.

Mr Trump Jr has said the lawyer had no meaningful information about Mrs Clinton.

Here are questions and answers about legal issues raised by this disclosure amid the criminal investigation by special counsel Robert Mueller into the Trump-Russia affair.

Q: What is collusion?

A: In general parlance, "collusion" means working together, usually in secret, to do something illicit. But the term has no defined legal meaning. Lawyers instead talk about the offence of "conspiracy".

Q: What is conspiracy?

A: In criminal law, conspiracy is generally an agreement by two or more people to commit a crime - whether or not they do. A powerful tool for prosecutors, conspiracy charges permit holding each conspirator responsible for illegal acts committed by others in the circle as part of the arrangement.

Q: Is the meeting enough to prove conspiracy?

A: The events made public in the past few days are not enough to charge conspiracy, said Mr Renato Mariotti, a former federal prosecutor. Still, he said, the revelations are important because if further evidence of coordination emerges, the contents of the e-mails and the fact of the meeting would help establish an intent to work with Russia on influencing the election.

"What this e-mail string establishes is that Don Jr was aware that the Russian government wanted to help the Trump campaign and he welcomed support from the Russian government," Mr Mariotti said.

Q: What else is needed?

A: Evidence of an agreement to violate a specific criminal statute - in other words, a conspiracy to commit a certain crime.

"Anytime you are talking about coordinating or collusion, you are talking about the possibility of conspiracy charges," said Mr Samuel Buell, a former federal prosecutor who teaches criminal law at Duke University.

"But conspiracy is not a crime that floats by itself in the air. There has to be an underlying federal offence that is being conspired to be committed."

Q: Was election law violated?

A: A federal law, Section 30121 of Title 52, makes it a crime for any foreigner to contribute or donate money or some "other thing of value" in connection with an American election, or for anyone to solicit a foreigner to do so.

Legal experts struggled to identify any precedent for prosecutions under that statute, but that phrase is common in other federal criminal statutes covering such crimes as bribery and threats, said election-law professor Richard Hasen of the University of California, Irvine.

Courts have held, in other contexts, that a "thing of value" can be something intangible, like information.

Mr Robert Bauer, an election-law specialist who served as White House counsel in the Obama administration, argued that this statute covers the Russian government's paying its spies and hackers to collect and disseminate negative information about Mrs Clinton to help Mr Trump win the 2016 election.

"There are firms in the United States that do negative research and sell it to campaigns," Mr Bauer argued. "There is no way to take information someone has compiled using resources and say it's just information and dirt. It's valuable information and counts as a contribution when given to or distributed for the benefit of a campaign."

But former federal prosecutor Orin Kerr, now a George Washington University professor, said the notion struck him as a stretch. "The phrase 'contribution or donation' sounds like a gift to help fund the campaign or give them something they otherwise would buy," he argued. "If that is the standard, that doesn't seem to be met, based on what we know so far, because this wasn't something that someone else could have gathered that was for sale in a market or would be otherwise purchasable."

Q: What about illegal hacking?

A: There is no public evidence, as things stand, of any clandestine discussions between Russian officials or surrogates and the Trump campaign about disseminating the e-mails of Democrats that US intelligence officials say Russia hacked.

In July 2016, however, the elder Mr Trump publicly urged Russia to hack Mrs Clinton's e-mails; his spokesman later insisted that was a joke.

But the Justice Department investigation is still unfolding. If it were to come to light that Russian officials did consult Trump campaign officials about the timing or tactics of the release of the stolen e-mails, that could raise the possibility of conspiracy charges under Section 1030 of Title 18, which bars unauthorised computer intrusions, specialists said.

Q: Are there other possibilities?

A: The unprecedented issues raised by the Trump-Russia affair have led criminal law specialists to delve into other, more creative theories. For example, the federal conspiracy statute also prohibits conspiracies to "defraud" the US by impeding the federal government's lawful functions.

Mr Randall Eliason, a former federal prosecutor who teaches at George Washington University, has argued that this could include a conspiracy to undermine a federal administration of a presidential election, although administering elections is largely a state government task.

Several legal experts cautioned that the public does not know everything Mr Mueller's investigation has uncovered and that it is not yet complete, so evidence of other potential crimes may emerge that have not yet been the subject of much discussion.