Vision and Light
The most fundamental tasks of adventuring- noticing danger, finding hidden objects, hitting an enemy in combat, and targeting a spell, to name just a few-rely heavily on a character's ability to see. Darkness and other effects that obscure vision can prove a significant hindrance. A given area might be lightly or heavily obscured.
The presence or absence of light in an environment creates three categories of illumination: bright light, dim light, and darkness.
Blindsight
Darkvision
Truesight