Short definition
The droplets of wine that slide down the inside of the glass after swirling, also called tears.
Swirl your glass and watch the rivulets that form on the sides — those are legs. Thick, slow-moving legs mean higher alcohol or residual sugar. Thin, fast legs mean lower alcohol. Despite what some claim, legs tell you nothing about quality. A cheap fortified wine has dramatic legs; a world-class Chablis barely shows any.