It is an existing defect in the system which 
does not cause any failure as the exact set of
conditions has never been met

Basically when one defect hides other defect
we call it fault masking for example:
If the "Negative Value" cause a firing of unhandled
system exception, the developer will prevent 
the negative values input.
This will resolve the issue and hide the
defect of unhandled exception firing.