Less trivially, a token stack is any set of one or more tokens which are not being held by a player. Token stacks are most commonly placed on a station, but may exist in other non-locations in certain circumstances.
Token stacks are endemic within the game and its derivatives, being used variously as a mechanism for token storage or token collection, as proxies and substitutes for players themselves, as launch points and amplifiers for translation manœuvres, and as gravitational attractors in compound gradient calculations, to name but a few.
The Fronsky diagram symbol for a token stack is a triangle with its vertex pointing towards the top of the page.
See also: token sink.