An acronym for Pot-Limit Omaha Hi/Lo 8-or-Better often used in both online and live card rooms.
PLO8 is an Omaha game played with four hole cards and five board cards. The PLO8 variety splits a high pot and a low pot if a player holds an Omaha valid hand of five cards all 8 or lower. Flushes and straights do not count against a player’s low hand and an Ace is the lowest card.
In Omaha hi-low split-8 or better (simply Omaha/8), each player makes a separate five-card high hand and five-card ace-to-five low hand (eight-high or lower to qualify), and the pot is split between the high and low (which may be the same player). To qualify for low, a player must be able to play an 8-7-6-5-4 or lower (this is why it is called “eight or better”). A few casinos play with a 9-low qualifier instead, but this is rare. Each player can play any two of his four hole cards to make his high hand, and any two of his four hole cards to make his low hand. If there is no qualifying low hand, the high hand wins (scoops) the whole pot. This game is usually played in the fixed limit version, although pot limit Omaha/8 is becoming more popular. A few low-stakes online tournaments feature no limit Omaha/8.
The brief explanation above belies the complexity of the game, so a number of examples will be useful here to clarify it. The table below shows a five-card board of community cards at the end of play, and then lists for each player the initial private four-card hand dealt to him or her, and the best five-card high hand and low hand each player can play on showdown:
In the deal above, Chris wins the high-hand half of the pot with his J-high straight, and Bryan and Eve split the low half (getting a quarter of the pot each) with 7-5-3-2-A.
For more: https://en.wikipedia.org/wiki/Omaha_hold_%27em