In beverage history, the word shrub has carried several meanings. For our purposes, it’s enough to say that a shrub is an acidulated beverage made of fruit juice, sugar, and other ingredients. Where things get complicated is that the acid varies by recipe; it can be either fruit juice or vinegar. Additionally, some shrub recipes are prepared using alcohol that steeps with the fruit, acid, and sugar. Finally, hardcore shrubbers make their own vinegar, using fruit juice, sugar, and wild yeasts from the air.
(via Cocktail 101: How to Make Shrub Syrups | Serious Eats: Drinks)
