Darkvision

Darkvision, also known as infravision, was a type of vision that many creatures had, in addition to "normal sight", that allowed them to see in darkness.

"So you sneak around in the dark? You know most everything but humans can see in the dark right? We all see you. Tiptoeing doesn't turn you invisible."

- The beholder Xanathar to a gloom stalker ranger.

Description
Darkvision allowed a creature to discern forms, as in normal daylight vision, but only in shades of gray. Monsters with this ability could only see in the dark up to a certain distance, typically around. Permanently subterranean creatures often possessed superior darkvision that allowed them to see in the dark at distances twice that far. Darkvision did not function in darkness that was magical in origin.

Despite the name "infravision", this form of vision did not allow creatures to see light that fell within the infrared spectrum; it did not involve heat-sensing, and so sources of light or heat did not affect it.

Creatures
Creatures known to have darkvision included drow, duergar, dwarves, svirfneblin, and many others.