opponent's mistake; error by the enemy
Used in competitive contexts (sports, games, business, military) to refer to a mistake made by the opposing side that benefits one's own side.
They capitalized on the opponent's mistake and turned the game around in one go.
We won the match due to the opponent's mistake.
It's important to attack without overlooking the enemy's errors.
We won today's game, helped along by the opponent's mistake.
The opposing team kept making mistakes, giving us more chances.
ミス is a generic, everyday word for any mistake, while 敵失 is limited to a mistake made by the opponent.
エラー is a loanword for an error, especially in baseball, without specifying whose side made it, unlike 敵失 which is always about the opponent's mistake.
Compound of 敵 (enemy, opponent) and 失 (loss, mistake). The exact origin is uncertain, but it likely emerged in modern competitive contexts such as sports and military analysis.