Aim Mode (Defiance)  

Defiance Wikibase™ Dictionary

On PC, the left-mouse button is Fire and the right-mouse-button is Aim. Aim focuses in on your target, activates the Scope if equipped, and engages Aim Assist (if enabled).

Aim Assist helps you by adjusting your aim slightly to target the mob you are most-closely aimed at. This may not be right at the sweet spot but it will be in the mob. This makes it possible to quick-shot fast-moving enemies if you depress Aim and Fire together, and is an important skill to learn (especially with weapons with a high Recoil).