Design Nomenclature / Form & Geometry
Watertight
Also called closed, solid, printable, sealed
A mesh that bounds a solid: closed, with no boundary edges, consistently oriented, and enclosing a definite inside. This is a fabrication requirement, not a topological property. A Klein bottle is a perfectly good manifold and is not watertight, having no inside; a closed mesh with self-intersecting walls is watertight by the letter and unprintable.
In practice
Used as a synonym for 'manifold' and it is not one — a model can be manifold and open, or closed and non-manifold. 'Solid' and 'closed' are the CAD words, and the slicer means all three at once.