Short definition
The year the grapes were harvested, printed on the bottle to indicate growing conditions for that specific year.
The vintage year on a bottle tells you when the grapes were picked, not when the wine was released. A hot year produces riper, fuller wines; a cool year yields leaner, more acidic ones. In regions like Bordeaux and Burgundy, vintage variation is dramatic — a 2015 and a 2013 from the same producer taste like different wines. Non-vintage wines (common in Champagne) blend multiple years for consistency.