Gold sink

A gold sink is any process in the game that makes gold (or any items that can be valued against gold) disappear from the game. To have gold sinks is required to maintain a stable in-game economy, because without gold sinks the constant influx of gold value from drops would result in rapid inflation.

As such, the main gold sinks in Guild Wars are the various NPCs who require payment for their services (crafters, skill trainers, merchants, etc.).

In more colloquial speech a gold sink is an item that has a very high price compared to the actual added functional value, i.e. especially "prestige" items like Obsidian armor, rare skin high-end weapons like Crystalline Swords, black dye, or miniatures. (Note that strictly speaking many of these are not gold sinks per definition, because the gold paid only moves from one player to another, but is not actually removed from the game.)