Non-destructive testing (NDT) is a testing and analysis technique used by industry to evaluate the properties of a material, component, structure or system for characteristic differences or welding defects and discontinuities without causing damage to the original part.
Destructive testing destroys or changes the part in some way such that even if it passes the test it is no longer fit for service. Examples might be tensile testing, 3 point bend test or macro sectioning. NDT does not destroy or change the part such that it is still fit for service if it passes the test.