- Were you looking for the formal fear check set of quests called the Proving Grounds?
A gear check is usually a boss in a raid instance that cannot be beaten without a certain quality of gear (often meaning average item level). Characteristics of a gear check (or gatekeeper) boss include some or all of the following:
- High damage output (may be high burst damage, AoE, or high sustained DPS on a single target)
- Short enrage timer
- Low amount of player skill (usually tank and spank, at the same time often a DPS race)
The game designers use these gatekeeper bosses to ensure enough of the raid members did pass through the intended content progression before continuing deeper into the instance.
The term 'gear check' is sometimes used in a derogatory manner due to the fact that after surpassing the requirement, gear checks can become easy to the point of silliness (for example, with Tier 5 gear, the Curator is dead after 70 seconds or less).
Usually, the end boss fight of a raid instance is not designed as gear check, but rather requires high levels of skill even after beating it many times (possible exception: 
Prince Malchezzar, though this was probably not the designers' intent, as the "safe spots" were removed in Patch 2.4).
For example Razorgore the Untamed (first boss of Blackwing Lair) requires a high amount of skill and coordination, but only a few key raid members are needed with good gear (on healers good gear may even be detrimental because of aggro issues). The second boss ( Vaelastrasz the Corrupt), however, hits very hard and is on a three-minute timer. The whole raid must deliver very high DPS, several tanks with good gear must take turns in tanking him, and even with well-equipped tanks, the healers are under extreme pressure to keep them up. Most of the raid members must have spent a lot of time in Molten Core to gear up sufficiently in order to beat Vaelastrasz.
Some famous 
Burning Crusade boss examples for gear checks are: Patchwerk, The Curator and Brutallus. An example from 
Wrath of the Lich King would be Festergut. Ultraxion from Dragon Soul is a good 
Cataclysm gear check example. Some consider Elegon from 
Mogu'shan Vaults to be a gear check. A more recent gear check boss is The Butcher in 
Highmaul.
Notes
- The term can also be used to describe the minimum gear some guilds will accept when recruiting new members. Usually, this applies to raiding guilds, but can apply to any guild. This can also be called a "Gear Score".
