Darkness
From PMDOWiki
Darkness is a natural feature of dungeons that restricts visibility of the floor. Tiles that have been previously revealed but are not currently within the player's sight will be shaded. Unrevealed tiles will be completely obscured.
Types of Darkness
There are five types of darkness. Additionally, darkness can be set separately for the floor ("Tile sight") and enemies ("Char sight") — this can make it so that the floor is lit but enemies are hidden, or vice-versa.