President Donald Trump rails against it all the time. But what is it? And does it even exist?

Trump's latest salvo against the shadowy forces he sees as ranged against him came after The New York Times published an opinion piece by someone identified as a "senior administration official." The anonymous author claimed to be a member of the "Resistance" inside Trump's administration, "working diligently from within to frustrate parts of his agenda and his worst inclinations."

"It may be a Deep State person that's been there a long time," Trump complained to the show "Fox and Friends," speculating on the identity of the author.

"The Deep State and the Left, and their vehicle, the Fake News Media, are going Crazy - & they don't know what to do," he tweeted, lumping his favorite villains together.

Like the "Deep State" itself, the origins of the term are shrouded in mystery.

Some political observers see the "Deep State" as the heir to the "military-industrial complex" warned about by former president Dwight D Eisenhower. Some have used the phrase to refer to the military power structures behind ostensibly democratic regimes in countries such Egypt, Pakistan and Turkey.

For some, the "Deep State" is the thousands of federal government employees who soldier on through changes in administration.

"The Deep State is, in fact, a very real thing," geopolitical analyst George Friedman wrote in The Huffington Post. "It is, however, neither a secret nor nearly as glamorous as the concept might indicate.

"This entity is called the civil service, and it was created to limit the power of the president."

The anonymous author of the piece in The New York Times lent some credence to this explanation, seeking to reassure readers there are "adults in the room."

"This isn't the work of the so-called Deep State," he or she wrote. "It's the work of the steady state."

Writing in The Guardian in April, Jack Goldsmith, a law professor at Harvard, said the "Deep State," such as it is, is "national security bureaucrats who use secretly collected information to shape or curb the actions of elected officials."

If the "Deep State" does indeed exist, leaks to the press -- Trump's "enemy of the people" -- are its currency.

"Since Trump was elected, unusually sensitive leaks of intelligence information designed to discredit him and his senior leadership have poured forth from current and former intelligence officials in the Deep State," Goldsmith said.