NDT (Non-Destructive Testing) refers to a variety of testing methods used to evaluate the properties or integrity of materials or components without causing any damage to them. These tests are essential in industries like aerospace, automotive, manufacturing, and construction to ensure safety, quality, and performance without compromising the object being tested. 

Each technique has its own strengths and limitations, and the choice of method depends on the material type, structure, and the type of defects being sought.