Blindsight

Blindsight is a special sense that lets a creature see within a set range without using its eyes.

A creature with Blindsight can see anything within the listed range (for example, 30 feet) that isn't behind Total Cover, even if it is Blinded or in Darkness.
Within that range, the sense ignores the Invisible condition: an invisible creature gains no benefit from being unseen against a creature using Blindsight.
It does not depend on light, so magical or nonmagical darkness has no effect on it inside the range.
Beyond the stated range, the creature relies on its other senses as normal.
Blindsight perceives nothing that ordinary sight couldn't if it weren't blocked - it can't see through walls or detect creatures behind full cover.

The exact range is given by whatever grants the sense (a spell, species trait, or monster feature).