Abstract (layout abstract)
Beginner
A simple outline of a chip part: its size and where its connections are, with the inside left out.
Novice
A stand-in for a cell or block that shows only its outline, its connection points (pins) and the areas wires must avoid. Placement and routing work with abstracts, usually in the LEF format, because they don’t need the inside.
Expert
The LEF view of a cell or macro: SIZE, PIN shapes by layer and OBS (obstructions). At tapeout every abstract must be replaced by the real layout of the same name; one that isn’t becomes an empty hole on the masks.