solera
From The Oxford Companion to Spirits & Cocktails
is a rank of barrels used for maturing wines and spirits where, as liquid is withdrawn from the oldest one (never entirely emptying the barrel), it is replaced by a like quantity from the next oldest, which is replenished from the next one, and so forth. See maturation.
This definition is from The Oxford Companion to Spirits & Cocktails, edited by David Wondrich (Editor-in-Chief) and Noah Rothbaum (Associate Editor).