What was the name of the Union ironclad that fought the CSS Virginia?

The Union ironclad that fought the CSS Virginia was the USS Monitor.

The Monitor was designed by Swedish-American inventor John Ericsson and commissioned by the Union in early 1862. Its most distinctive feature was a rotating armored gun turret, which allowed its weapons to fire in multiple directions without turning the entire ship.

On March 9, 1862, the Monitor met the Virginia at Hampton Roads, Virginia. The Virginia had been rebuilt from the former USS Merrimack, which explains why the Confederate ship is sometimes mistakenly called “Merrimack.” The two ironclads fought for several hours without either ship being decisively defeated, in what is generally regarded as the first battle between ironclad warships.

The clash ended without a clear tactical winner, but it demonstrated that armored steam-powered ships had made traditional wooden warships obsolete. The Monitor later sank in a storm off Cape Hatteras in December 1862.