Masked defect means:
Masked defect is the defect which is hiding the other defect, which is not
detected. Means if there is a existing defect which is not caused (found) to reproduce another defect. It means the other defect is masked with the previous defect.
Its very best example is
If you are testing the Help of any
application. If there is a defect in one link say Add Customer. But this defect was not found by QA and it is went to the live. Then there is one defect residingin the Add Customer link say the link is not working on the Add Customer page (Say there is bug in help of adding cell no in the help in the Add Customer)
Then this Add cell no is a masked defect. It means it is been masked with the Add Customer defect.