The terminator, in drawing and astronomy, refers to the boundary between the illuminated area (form light) and the shadowed area (form shadow) of an object.

Key points about the terminator:

1. Location: It appears where light rays become tangent to the surface of the object.
2. Shape: The terminator's shape reveals information about the object's form and the direction of the light source.
3. Distinction from core shadow: While the terminator marks the start of the form shadow area, it is distinct from the core shadow. The core shadow is the darkest part of the shadow, often adjacent to the terminator but inside the form shadow area.

Some instructors use terms like "shadow line", "bedbug line", or "shadow edge" which may encompass both the terminator and the core shadow. The terminator can be useful for initially blocking in the major light and shadow areas in a drawing.