The solera system is a method of aging and blending fortified wine or spirits, most famously sherry, by fractional blending across a stack of barrels of different ages rather than aging a single batch to a fixed point.
The oldest liquid sits in the bottom barrel (the solera); when some is drawn off for bottling, it's topped up from the barrel above (a criadera), which is in turn topped up from the barrel above it, and so on up to the youngest barrel at the top. Because each barrel always contains a blend of vintages, no bottle is ever a single-year product, and the system produces a consistent house style year after year.
Solera aging originated in the Jerez region of Spain for sherry production and is also used for Spanish brandy and some rums from Latin America and the Caribbean.