The spellcasting rules for areas of effect state:
A spell's description specifies its area of effect, which typically has one of five different shapes: cone, cube, cylinder, line, or sphere. Every area of effect has a point of origin, a location from which the spell's energy erupts. The rules for each shape specify how you position its point of origin. Typically, a point of origin is a point in space, but some spells have an area whose origin is a creature or an object.
A spell's effect expands in straight lines from the point of origin. If no unblocked straight line extends from the point of origin to a location within the area of effect, that location isn't included in the spell's area. To block one of these imaginary lines, an obstruction must provide total cover.
Notably, square is not one of the shapes defined, yet there exist several spells which have a square area of effect, such as entangle or Evard's black tentacles.
The spell grease tells us in its description:
Slick grease covers the ground in a 10-foot square centered on a point within range.
So what is the point of origin of a square area of effect when it is not specified in the spell description?