Senses

The Senses entry notes a monster's passive Wisdom (Perception) score, as well as any special senses the monster might have. Special senses are described below.

Blindsight

A monster with blindsight can perceive its surroundings without relying on sight, within a specific radius.
Creatures without eyes, such as grimlocks and gray oozes, typically have this special sense, as do creatures with echolocation or heightened senses, such as bats and true dragons.
If a monster is naturally blind, it has a parenthetical note to this effect, indicating that the radius of its blindsight defines the maximum range of its perception.

Darkvision

A monster with darkvision can see in the dark within a specific radius. The monster can see in dim light within the radius as if it were bright light, and in darkness as if it were dim light. The monster can't discern color in darkness, only shades of gray. Many creatures that live underground have this special sense.

Tremorsense

A monster with tremorsense can detect and pinpoint the origin of vibrations within a specific radius, provided that the monster and the source of the vibrations are in contact with the same ground or substance.
Tremorsense can't be used to detect flying or incorporeal creatures. Many burrowing creatures, such as ankhegs, have this special sense.

Truesight

A monster with truesight can, out to a specific range, see in normal and magical darkness, see invisible creatures and objects, automatically detect visual illusions and succeed on saving throws against them, and perceive the original form of a shapechanger or a creature that is transformed by magic. Furthermore, the monster can see into the Ethereal Plane within the same range.