Stuck-at fault
Beginner
A pretend flaw where one wire acts as if it were stuck at 0 (always off) or at 1 (always on).
Novice
The classic fault model: one wire in the circuit is assumed to be permanently 0 (stuck-at-0) or permanently 1 (stuck-at-1), whatever drives it. Every gate input and output has both possibilities.
Expert
Single (one fault at a time), permanent, at gate pins. Simple and a good proxy for many shorts and opens, but blind to timing defects and to many defects inside cells. Lists are shrunk by equivalence collapsing before test generation.