Dont go with elaborate answers just keep it to the point explain what is a "defect". Defect is a flaw or an error in an application that is restricting the normal flow of an application, blocking activity on what a system needs to perform. If testers find a mismatch in the actual and expected behavior of an application in the testing phase then they call it as a Defect. life cycle of defect : new --> assigned --> resolved -->--reopen/ verified ---> closed