In WARFRAME, many item drops from Drop Tables are tagged by different rarity tiers for purposes of determining their drop rates or as a way to label their intended rarity.
There are four official rarity tiers used by the game. In order of increasing rarity, these are: "Common" (bronze or brown ◼ ), "Uncommon" (silver or gray ◼ ), "Rare" (gold or yellow ◼ ), and "Legendary" (platinum ◼ or purple ◼ ). In most cases, rarity tiers and actual item rarity are used interchangeably to describe an item's rareness, but a distinction needs to be made if players want to use more precise language to describe "Rarity" as a mechanic or "rarity" as a player's measure of drop frequency or classification.
Note how the game classifies item rarities may not match the actual rarity weights that these items use internally. There are also sometimes biases introduced so that not all items tagged with the same rarity weight have their drop chances distributed evenly.
As a measure of drop frequency (Rarity Classification)[]
This is how most players understand rarity from a non-technical perspective. Items labeled "rare" or have "rare" rarity coloration/visualizations like Intensify, Arcane Rage, and Orokin Cells will tend to drop less often than their "uncommon" or "common" counterparts. However, not all rare items share the same drop chances or drop experiences because of factors dependent on developers and players:
Drop table chances of enemies
Number of different items that occupy a drop table (the more items that occupy a particular rarity weighting, the less chance that players will get a particular item)
Number of sources that a particular item can drop from
Bias applied to certain items within a drop table
Spawn rate of enemies
Enemy kill rate
Mission completion rate (i.e. how fast mission objectives are completed for rewards)
Pity system as in the case of weapons obtained from Lich System
As a tag[]
Some stats or properties are tied to the item's rarity. For example, the amount of Endo required to upgrade mods, the amount of Endo you get from dissolving mods, and their trading tax are tied to the mod's rarity.