Attunement
A limiting system primarily designed to restrict the number of a kind of item a specific character can use at that same time. In addition, it usually restricts the number of characters that can use a specific item. Attunement should always be a strategic choice made by the player (or players in a group game), and not a simple class, race, or level requirement of an item.
Note: If deattunement is not allowed, then the goal of the system is probably to remove used equipment from the economy, not to put a check on character power.
The first restriction, where each character has a limit on the number of attuned items, is essentially the equipment version of concentration. This prevents a single player from loading up on all the best equipment, possibly reaching game-breaking levels of power.
The second restriction, where each item can be attuned to a limited number of characters (typically only one), prevents an item from being convienently shared or swapped around during combat as the player pleases.
Attuning and deattuning typically have some cost (money / time) or are only allowed under certain conditions, like being at a specific location.