Bitters

A concentrated botanical infusion added by the dash to season a cocktail.

Bitters are the salt and pepper of the bar: high-proof, intensely flavored, and used in tiny quantities. Angostura and Peychaud's are the two 19th-century survivors still in daily use; a modern bar carries a dozen more (orange, chocolate, celery, cardamom). Without bitters, a cocktail is legally just a sling.