Like "Just A Rather Very Intelligent System (J.A.R.V.I.S. )", does U.L.T.R.O.N stand for something in comics or movies in Marvel Cinematic Universe?
Out-of-universe - it's a name, rather than an acronym.
A mix of 'Ultroids' and 'tron'
In an interview with The Hollywood Reporter, Roy Thomas, who co-created Ultron, answered:
Where did the idea for Ultron come from?
I never considered myself very good at making up names. Some of the first creatures I made up fighting The Avengers were part of a group called the Ultroids. I've always liked that tron ending. I had recently made up something called a psychlotron [a brainwashing device]. So I liked that tron ending, and Ultron just came as a good name.
It's not an acronym.
Name "Ultron" actually came from the ancient times:
The inspiration behind the name "Ultron" actually came from the Roman god of war Mars whose full name (in terms of the version chosen for Ultron's inspiration, at least) is "Mars Ultor". The literal translation of Mars Ultor is actually "Mars the Avenger", which essentially means that Ultron's name is a derivative of the word "Avenger" - the name of the team to whom he has been a robotic thorn in the side for many a year since his first appearance. Mars Ultor is also the name of a cult of Mars' followers, which is also interesting given that Ultron has his own cult of followers - the "Sons of Yinsen" - in the Marvel comic books.