The situation that occurs when the actual game hold is greater the amount a player would lose given perfect play (Max-EV strategy). Most-- if not all-- casino games Over-hold as most players do not play perfectly nor follow a max EV strategy. For many new video poker games the Over-hold is significant as the optimal strategy is difficult to master, often non-intuitive, and differs significantly from older games.
Following a pre-determined strategy perfectly, without mistake or deviation, in any way. In most cases, Perfect Play refers to play following a simplified Max-EV strategy.