This is one of the most asked interview question for experienced people. Here we go with it:

If the client/customer/end user finds a defect while using the released Application/Product.Then it is called as Defect Leakage or bug leakage.

OR in other words

After the release of the application to the client, if the end user gets any type of defects by using that application then it is called as Defect leakage. This Defect Leakage is also called as Bug Leak.