What was the Revolution of 1800, and in what way was it a revolution?

"The Revolution of 1800" generally refers to the election of Thomas Jefferson as President of the United States in 1800. Jefferson was the third President, after Washington and Adams. The "revolution" he brought was not a literal revolution like the recent Revolutionary War, but it is sometimes referred to that way because of the sweeping and permanent changes Jefferson made to the role of government in the United States.

It was also the first time...

"The Revolution of 1800" generally refers to the election of Thomas Jefferson as President of the United States in 1800. Jefferson was the third President, after Washington and Adams. The "revolution" he brought was not a literal revolution like the recent Revolutionary War, but it is sometimes referred to that way because of the sweeping and permanent changes Jefferson made to the role of government in the United States.

It was also the first time the United States transitioned in governance between two political parties, and the revolutionary thing about it was really that it wasn't violent, and involved a seamless transition of power by democratic means---something the world had not seen in a major country for hundreds of years.

Washington and Adams had both been Federalists, who favored a strong, centralized government; but Jefferson was a Democratic-Republican, who favored a decentralized government with maximal liberty for states and the people. (Both the Republican Party and the Democratic Party of today are offshoots of Jefferson's Democratic-Republican Party---that's how influential he was.)

One of the changes Jefferson made to the Presidency is still visible today; he eschewed formal titles and sought to treat everyone as equal, at least nominally. While in the UK you would address the Queen as "Your Majesty", in the US you address the President as simply "Mr. President"; that was Jefferson's doing. It was also largely due to Jefferson that we have no official noble titles in the US, no Lord this or Baron that.

More than anyone else, it was Thomas Jefferson who laid the foundation of American government as we know it today; and that would not have been possible had he not been elected President in 1800.

That sounds pretty revolutionary to me.