A stash is a collection of three trios all the same color, 15 pyramids total.
Some games require just one stash per player, while others require a fixed total number of stashes. The "traditional" colors used in four-stash sets are red, yellow, green, and blue.
The original resin Icehouse pieces (The First 100) were sold by the stash, with all 15 pieces of the same color, five of each size. Later products came packaged as complete Icehouse sets. Monochrome stashes returned to the market in 2002, and were discontinued in 2006 with the arrival of Treehouse.
Looney Pyramids are now sold in one-stash boxes based on the Treehouse model: each contains a spectrum of five colors, so that a set includes one pyramid of each size-color combination. The Rainbow Stash has red, yellow, green, blue, and black; the Xeno Stash has purple, clear, orange, cyan, and white.
In the rules for Zark City, "Stash" is used to refer to each player's set of 3 trios.