A player who plays very predictably. ABC players follow the same betting patterns by making consistently making similar plays.
A predictable player, he follows the rules of the game to the T. While he does not risk much and wins easily with lower stakes, experts can run him down easily.
An "ABC player" is someone very predictable. Such a player usually reads a few poker books follows them exactly. These players always play hands in the same manner and are pretty tight. Such players often win at lower stakes and loose games but are easy to take advantage of by more experienced players.