An Abstract Game is an abstract description of a game which other games can be “reduced” to, and may or not be playable in its abstracted state.
Reducing one game to another means removing game rules and mechanics while retaining the “essential” dynamics and experience of the original game, until the game resembles or even becomes the reduced game. For example - the game Rock-Paper-Scissors-Lizard-Spock can be unsurprisingly reduced to Rock-Paper-Scissors, as outside of the increased choice amount, both are still a short game where each player makes an arbitrary selection, and assuming the other player has not made the same selection, have a 50% chance of success or failure.