What is treason, exactly?

In U.S. law, it’s a very narrow set of crimes — so narrow that only a couple of dozen people have been charged with it since the dawn of the republic.

But it’s serious business, so much so that it’s the only crime specifically described in the Constitution itself: “Treason against the United States,” says Article III, “shall consist only in levying war against them, or in adhering to their enemies, giving them aid and comfort. No person shall be convicted of treason unless on the testimony of two witnesses to the same overt act, or on confession in open court.”

The word “only” — that treason shall “consist only” of certain specified acts — was crucial to the drafters of the Constitution. In England, treason was “abused as a way of getting at the King’s enemies,” Columbia Law School’s Richard Briffault told Business Insider. “And so I think the special requirements of proof, and the specific definition of treason … was a way of narrowing the definition of what treason is.”

You wouldn’t know that from current political discourse.

“Treason” has been tossed around since the early days of the investigation into Russian meddling in the 2016 election, often by critics of President Trump who seem to think that meetings with politically connected Russians to get dirt on Hillary Clinton would qualify as such. (They wouldn’t.)

Trump, too, has his own definitions. In a January interview with the Wall Street Journal, he said texts between two FBI agents that were critical of him were “treason right there.”

On Monday, the president added yet another offense to the ever-expanding sphere of supposedly treasonable conduct, as The Washington Post reported. In a speech in Ohio, Trump accused Democrats of being “un-American” and “treasonous” when they didn’t applaud as he touted positive numbers about black and Hispanic unemployment in his State of the Union address.

“Even on positive news like that, really positive news like that, they were like death, and un-American,” Trump said during an off-script moment. “Somebody said ‘treasonous.’ I mean, yeah, I guess, why not? Shall we call that treason? Why not? I mean, they certainly didn’t seem to love our country very much.”

None of this, of course, is treason, a crime punishable by life in prison or even execution.

The United States has charged about 30 people with treason in all of its history. Part of the reason is that the crime applies specifically to declared “enemies” of the United States, meaning nations or organizations with which the country is engaged in open war.

Over the years, those enemies have included people who partook in the Whiskey Rebellion, which culminated in a 1794 violent uprising over the whiskey tax imposed by what was then a fledgling federal government, as well as a handful of Americans who defected to the Axis Powers during World War II.

Treason prosecutions have nearly disappeared since 1945. That year, the U.S. Supreme Court raised the bar for proving treason, over objections from the government that a more lenient interpretation was necessary during wartime, as the National Constitution Center has noted. “The treason offense is not the only nor can it well serve as the principal legal weapon to vindicate our national cohesion and security,” the court wrote in Cramer v. United States. The justices indicated that non-treason charges such as espionage might be a more appropriate option.

Sen. Jeff Flake (R-Ariz.) joined his Democratic colleagues on Feb. 6, in criticizing President Trump for labeling Democrats as "treasonous" and "un-American." (Reuters)

When John Walker Lindh, an American citizen, was captured in Afghanistan while fighting with the Taliban in 2001, prosecutors mulled charging him with treason but instead indicted him on other counts, including conspiracy to support terrorists. He ended up pleading guilty to two charges in exchange for a 20-year prison sentence.

In the first such case since World War II, the government in 2006 filed treason charges against Adam Gadahn, an American living in Pakistan at the time, after he made propaganda videos for al-Qaeda in which he vowed attacks on U.S. soil. Gadahn was killed in 2015 in a CIA drone strike authorized by President Barack Obama.

A citizen doesn’t have to take up arms to commit treason against the United States. Sometimes words are enough. In one famous case, Iva Toguri D’Aquino, better known as “Tokyo Rose,” was tried for treason after she broadcast propaganda in Japan during World War II. After being convicted in 1949, she was pardoned by President Gerald Ford. A similar World War II case involved a U.S. pilot who defected to the Waffen SS and worked as a propaganda broadcaster.

When Jane Fonda visited Hanoi in 1972, some in the United States rallied to have her tried for treason under the same theory. The Nixon administration declined because officials thought it would generate too much negative publicity as they tried to wind down military operations in Vietnam, as Henry Mark Holzer, professor emeritus at Brooklyn Law School, told the San Francisco Chronicle.

Indeed, treason has been tailored specifically to avoid trampling on free speech, so people like Fonda — and certainly Trump’s Democratic critics — are robustly protected.

“The definition of treason has always been defined narrowly in the United States because it has such great potential for criminalizing political speech and dissent,” George Washington University law professor Jonathan Turley told The Post in 2016. “The U.S. Courts recognized early that treason was not only narrowly defined historically but had to be confined to be consistent with the First Amendment.”

Trump’s critics have raised “treason” claims almost as recklessly as he has. Some say the president and his aides committed treason by allegedly colluding with the Kremlin to tilt the election in his favor. Sen. Tim Kaine (D-Va.) suggested that Donald Trump Jr. may be investigated for treason after meeting with a Russian lawyer to get dirt on Hillary Clinton. Former White House chief strategist Stephen K. Bannon called the same meeting “treasonous,” then sought to clarify that he was talking about Trump’s former campaign manager, Paul Manafort.

Such loose language about treason might surprise and alarm those who wrote the Constitution in 1787.

Abuse of the word and the crime itself are “great sources of danger and persecution on the part of the government against the citizens,” said James Wilson of Pennsylvania, a key drafter of the document.

“History informs us that more wrong may be done on this subject than on any other whatsoever.”