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Aim is a statistic (attribute) in XCOM 2 that affects the chance to hit a unit, in %.
It is an inherent stat, meaning units have a fixed Aim value. For ADVENT and alien units the Aim stat is affected by the game difficulty (lower for Rookie and increased for some higher difficulties, except for few units with a fixed Aim stat across all difficulties or a lower value for Veteran difficulty); advanced and elite enemy unit types usually have increased Aim compared to their lower rank types, but standard-only units do not increase their stats with the game progression (all aliens, except for Spectre which has tier-2).
For XCOM soldiers the Aim value and its increase progression varies by Classes. The Aim can be permanently increased, like most other stats, via Covert Actions. And it can also be temporary increased by a successful ADVENT Security Tower hack reward "Targeting"/"Squad Targeting".
The Hit chance is primarily derived from the Aim. The Hit chance is affected by Cover, Defense and other bonuses/penalties - these will be added or subtracted to/from the Hit chance. The Aim parameter is listed at the very top of the Hit calculation formula in the GUI screen section.
Some abilities/attacks are based off the Aim stat, e.g. Demolition (it only has +10% hit chance on top of the Aim stat) is unaffected by high ground hit chance bonus, cover, etc. Whereas some other abilities/attacks are guaranteed to hit ignoring Aim and Defense parameters, e.g. Hail of Bullets, Rend, grenades/bombs/mines, psionic attacks.
In the right context the term Aim can be used to mean the Hit chance (in a gameplay or on this wiki, as of now, before proper edits are made), however, these are distinct parameters and should be treated accordingly.