Stack shape terminology
Localized stack surface features
A floor is a completely flat area of the stack surface which has length greater than 1. Floors may be referred to by their width, e.g. "2-floor," "3-floor," etc.
A slot is a local depression in the stack surface characterized by its ability to be eliminated and converted into a floor feature with a single polyomino placement. A slot does not have any overhanging blocks above it, and the elevation of its immediately left- and right-neighboring blocks are equal.
Slots can be considered as the negative space counterpart to each polyomino present within the game (usually, the 7 tetrominoes). Upon filling them with the corresponding polyomino, the local stack surface feature becomes a floor.
A cistern can be described as a floor which has a relative local elevation of -1, meaning that its elevation is equal to the elevation of its immediately left- and right-neighboring blocks, minus one.
A notch is the special case of a cistern wherein the width is equal to 1.
A pillar is any local protrusion whose width is equal to 1 and whose relative local elevation is 1 or greater, and whose immediately left- and right-neighboring blocks' are at an equal elevation.
Pillars may be referred to by their height, e.g. "2-pillar," "3-pillar," etc.