Elo is a rating system used to calculate the relative skill levels of players in two-player games, such as chess. Developed by Arpad Elo, a Hungarian-American physics professor, the system assigns a numerical value to each player, which changes based on the outcome of matches. Winning against a higher-rated opponent results in a larger rating increase, while losing to a lower-rated player leads to a more significant decrease. The Elo system is widely adopted in various competitive games and sports, including chess, table tennis, and video games. It provides a standardized way to rank players and helps to create balanced matchups, making competitions more engaging and fair.