weakness

Weakness describes a lack of strength or a character flaw. It also refers to something bad for you that you cannot resist. (noun)

When a beam is not strong enough to hold any weight, this is an example of a time when the weakness of the beam prevents it from holding weight.
When you are short-tempered and quick to anger, this is an example of a weakness.
When you cannot resist chocolate even though it is bad for you, chocolate is an example of your weakness.

See weakness in Webster's New World College Dictionary