The term 'Snakes on a Plain' is a reference to the classic children's game, Snakes and Ladders. The game is played on a board with numbered squares, and players move their pieces according to the roll of a die. The object of the game is to reach the end of the board before any other player. The term 'Snakes on a Plain' refers to the snakes that appear on the board, which cause players to lose progress and have to start over. The crossword solution for this term is ASPS, which stands for 'adders and snakes on a plain'.