Donald Trump's longtime lieutenant has pleaded guilty to tax evasion but thanks to a deal with prosecutors, he is unlikely to serve more than 100 days in prison. 

Allen Weisselberg took the deal on Thursday, pleading guilty to 15 counts, including tax fraud and larceny.

Here's what it could mean for the Trump Organization. 

What did Weisselberg agree to?

The numbers guy did the maths.

Weisselberg, 75, could have faced years in prison if he went to trial and failed to beat tax evasion allegations.

But if he agreed to testify in an upcoming trial of the former president's company, he would probably serve no more than 100 days.

Now he's potentially going to be the star witness against the Trump Organization in a trial over what prosecutors say was a "sweeping and audacious" scheme by the company to help top executives, including Weisselberg, avoid taxes on perks like luxury cars and rent-free apartments.

What were he and the Trump Organization accused of doing? 

Federal authorities said Weisselberg, the Trump Organization's longtime chief financial officer, failed to pay taxes on more than $US1.7 million ($2.45 million) in compensation.

Among those perks: The Trump Organization paid the rent on his Manhattan apartment, covered private school tuition for his grandchildren, leased Mercedes-Benz cars for him and his wife, gave him cash to hand out as holiday tips and paid for flat-screen TVs, carpeting, and furniture for his winter home in Florida.

Weisselberg's son also didn't have to pay rent, or paid below-market rent, while living in Trump-owned apartments.

So after all that, is he still the Trump Organization's CFO? 

Weisselberg is still employed by the Trump Organization, but his title was changed to senior adviser after the July 2021 indictment.

In the words of Weisselberg's lawyer, it was time to put an end to the years-long "legal and personal nightmares it has caused for him and his family".

The Trump Organization seemed to have no hard feelings, calling Weisselberg a "fine and honourable man" who has been "harassed, persecuted and threatened by law enforcement". 

What are the terms of the plea deal?

A judge has agreed to sentence Weisselberg to five months at New York's infamous Rikers Island jail complex.

With good behaviour, he'll be eligible for release after little more than three months.

He'll also have to pay nearly $US2 million and spend five years on probation.

Crucially, though, he must testify truthfully when the Trump Organization goes on trial in October.

Weisselberg won't be formally sentenced until after the trial.

Until then, he remains free on bail.

Will the Trump Organization also take a deal? 

In its statement, the company said it had done nothing wrong and would "look forward to having our day in court".

Both Weisselberg and the Trump Organization initially pleaded not guilty to the charges, maintaining the perks were standard for companies and the investigation was politically motivated.

Weisselberg's agreement to testify, though, could substantially harm the company's defence.

That could increase pressure for it to resolve the case without a trial.

What does the Trump Organization actually do? 

It's a business entity through which Mr Trump manages his many entrepreneurial affairs, including his investments in office towers, hotels and golf courses, his many marketing deals and his television pursuits.

It runs golf clubs and hotels, collects checks from companies renting offices, and charges licensing fees to buildings and others for bearing the Trump name.

What does this mean for Donald Trump?

It isn't clear.

The Republican former president isn't charged in the case.

Prosecutors haven't alleged any personal misconduct by Mr Trump.

If his company is convicted of a crime, though, it could lead to steep fines or complicate future business deals.

A trial starting in October, right before the midterm elections, could also produce new revelations about the company's business practices — although Mr Trump's supporters might not care.

He's said the entire case is being cooked up by Democrats to damage him politically.