The term "The Double" in football denotes the winning of the nation's top league competition and the top club competition by a football club in a single season. Doing "the double" is a very prestigious feat in football and is not a very easy thing to do.
In England, a team that wins the Barclays Premier League and the FA Cup in a single season is said to do the double. This is often referred to as the League and Cup Double. In Spain, winning La Liga and the Copa del Rey is referred to as doing as double whereas in Italy winning the Italian Serie A and the Coppa Italia is said to be doing the double.