Short definition
A label term suggesting higher quality or longer aging, though its legal meaning varies dramatically by country.
"Reserva" on a Spanish Rioja means the wine aged at least three years, including one in oak. "Riserva" on an Italian Chianti means two years minimum. But in the US, Australia, and most New World countries, "Reserve" has no legal definition — anyone can print it on any bottle. Treat it as meaningful in regulated European regions and as marketing elsewhere.