An alcoholic beverage consisting of equal parts of lager beer and lemonade. In Northern Germany, it is also known as an Alster (short for "Alsterwasser") or Potsdamer. In German, or at least its Bavarian dialect, Radler literally means cyclist.
In the Bavarian state of Germany, a mix of Weißbier (wheat beer) and lemonade is called a Russ.
The generic English name of beer plus lemonade is shandy.
In France it is called a Panaché.
Radler was invented by the Munich gastronomer Franz Xaver Kugler when in September 1922 approximately 13,000 cyclists visited his tavern. On this particular day his beer started to run out, so he mixed the remaining beer with lemonade and pretended he created the Radler especially for the cyclists so that they could drive home without the risk of falling off their bicycle. During the summer months, radler is still very popular in Bavaria and the rest of Germany, due to its reputation of being a thirst-quencher.
See also: List of cocktails