Presumably, the original, old terminal is "Terminal 1".
The way the (2) new terminals were built, they were spaced far apart to avoid congestion. If and when traffic at ONT reaches or exceeds 10 million pax in two consecutive years, a third new terminal will be built in the space between the two new ones. That will become terminal three. In order to avoid future confusion with the terminal numbering, it was decided that number three would be skipped for the present.