The Omnimatrix (often referred to as the Omnitrix) is one of Azmuth's creations and the successor of the Ultimatrix.
The Omnitrix was first mentioned by Azmuth in Map of Infinity. He was planning on giving it to Ben, but it was not complete, and Ben's maturity needed to increase before he would be allowed to use it.
Azmuth, by the events of The Ultimate Enemy: Part 2, had completed the Omnitrix and claimed that he was working on it ever since Ben got the original Omnitrix (approximately six years ago). In the same episode, he gave it to Ben because he deemed him worthy of wielding it after removing the Ultimatrix from the latter's wrist.