A beverage flavored with lemon juice.
Escoffier's "Le Guide Cilinaire" has this basic recipe for lemonade :
“Dissolve half-lb. of loaf-sugar in one quart of filtered water.
Add the juice and the zests of the rinds of two lemons, and let
infusion proceed in the cool for three hours. Pass the whole
through a fine strainer ; add one syphonful of seltzer water, and
serve with a thin roundel of lemon in each glass.”
Many variations of this exist including using soda instead of water, making it without sweetener, or adding other fruit flavors. The lemon rind that Escoffier recommended is rarely included.