Trinket

Were you looking for the Trinkets slot that is one of the Equipment slots?

Trinket refers to a type of armor worn in the trinket equipment slot. Trinkets are also called devices, gadgets and other references.

Trinkets are itemized unlike any other type of armor. Whereas most high level armor will give moderate bonuses to four or five of the seven or eight stats useful to your particular class, a trinket will generally provide very large bonuses to just two stats. Typically, one bonus will be a flat always-on bonus, and the other will either be a use effect (that you must activate) or a proc (which will occasionally activate whenever you attack or are hit).

In a PvP context, "to trinket" refers to using a PvP trinket.

Info

A character can have up to 2 of these items equipped at a time.
Virtually no trinkets are available before about level 50. Exceptions include PvP trinkets and heirlooms, as well as some BoP crafted items.
With rare exceptions, trinkets are obtained as quest rewards, as boss drops, through your professions, as PvP rewards or by purchasing with emblems. They are almost never available on the Auction House.
Engineers can make several different kinds of trinkets, but most can only be used by other engineers.
Alchemists can make several blue-quality trinkets at 400 skill, as well as the [Philosopher's Stone], which is necessary for transmutations.
The only trinket Enchanters can make is [Smoking Heart of the Mountain].
Jewelcrafters can make several trinkets, known as figurines, though all bind on pickup so crafters can only make trinkets for their own use.
Only blacksmiths can obtain the quest trinket, [Signet of Expertise].
There are many class-specific trinkets, such as the hunter's [Devilsaur Eye] and [Devilsaur Tooth] and the Zul'Gurub trinkets. There are none in content, however.
Trinkets are normally taken into very different consideration than weapons and armor, as very few have actual stats on them. Instead, they often carry passive effects or can be used to produce a unique effect for a brief period with a cooldown between uses. Trinkets are often very situational, and a good player will often carry around many of them.
After equipping a trinket, there is a 30-second delay before it can be used and any passive effect will start after the hidden internal cooldown, if any.
Using a trinket often activates a shared cooldown on trinkets with similar effects.
Using a trinket does not activate the Global Cooldown.
Trinkets cannot be equipped when in combat.
Trinkets in an action bar have a green border when equipped.
DPS classes will almost always want to have a macro that activates all of their on-use trinkets at once, as well as temporary damage-boosting abilities like [Icy Veins] or [Avenging Wrath].