Definition of pot limit
"Pot limit" means that the poker variant in question does not allow, as in no-limit poker variants, to put one's entire stack into play at any time. So there are betting limits that are calculated relative to the size of the pot. For example, in no-limit hold'em, if you have a 100??? stack, you can bet it on the flop, even if the flop pot is 10???. In no-limit variants, the size of your stack will determine your maximum bet, not the size of the pot. In a pot limit variant, you would not be able to bet more than 10???. The pot limit type of poker is most popular in Omaha.
Why do we play mostly Pot Limit Omaha?
If you see a poker variant that is pot limit, chances are it will be Omaha. The reason being that in Omaha, the preflop hands are closer together than in Hold'em. While you can have a huge advantage between two hands in Hold'em (an 80%/20% for example), this is never the case in Omaha. The big equity gaps between two hands will be more like 65%/35%. For this reason, a limit is imposed on each bet.