flaw
Definitions
A mark, fault, or other imperfection that mars a substance or object.
A fundamental weakness or imperfection in a person's character or in a plan, system, or theory.
Examples
The only flaw in his plan was the lack of a backup strategy.
She discovered a minor flaw in the diamond that reduced its overall value.
Despite his character flaw of being overly stubborn, he was a very loyal friend.
Synonyms
defect imperfection fault blemish weakness shortcoming error