What is a Usability bug?

What is Usability

Software is usable when it is effective, efficient and the usage is satisfying.

Effectie means: I can do what I want to do.
Efficient means: I can do what I want to do in a convenient and time saving way
satisfying usage menas: The software does not frustrate me, because I cannot find what I am looking for, the usage is inconvenient, etc.