Item Level

Item Level is a recurring gameplay element in the MMORPG entries in the Final Fantasy series. Broadly speaking, item level measures how powerful a piece of equipment is. Item level is not necessarily the same thing as the character level at which a given piece of equipment can be equipped, although at lower levels it usually is. Item level appears in the item's stat box. Progression in item levels allows characters to continue to become more powerful even after they have reached the game's level cap. Because the progression to become ever more powerful is a major component of the drive to play MMORPGs, item level is thus a vital gameplay concept.

Final Fantasy XI
Item level was introduced in Final Fantasy XI: Seekers of Adoulin, when the level cap was already at level 99 (the highest it will ever go). Thus, all pieces with an explicit item level stat are equippable at level 99 and have an item level of 100 or greater (the current maximum item level is 119).

Besides serving as a measuring stick for players to compare their strength against the challenge of a given piece of game content, item level has powerful in-game statistical effects. A player's overall strength is determined by the average item level of a player's visible gear (accessories are not visible and do not have an item level stat). Thus, the /check command monster difficulty messages are determined by the player's average item level vs. the monsters level. More importantly, the same method is also used to determine how many Experience Points a defeated monster is worth. A number of other level-dependent parameters are based off of average item level as well:
 * The strength of a Beastmaster's jug pets.
 * The strength of a Dragoon's Wyvern.
 * The strength of the NPC companions summoned by Trust Magic.

Final Fantasy XIV
Item level was introduced in Final Fantasy XIV when the game's level cap was level 50. Thus, item equip level and item level diverge at this point. Besides serving as a measuring stick for players, item level also serves as a method of gating content, as instances and such generally have a minimum item level required to enter.