Discontinuity – An interruption (Gas hole, cold shut, crack, forging lap, seam, inclusion, porosity, etc.) in the normal physical structure or configuration of an product or part.
It may or may not be a defect
Defect – a discontinuity that interferes with the usefulness of a part or exceeds or acceptability limits established by applicable Standard Specifications & Code
Not all discontinuities are defects
Surface discontinuity – a discontinuity open to the surface
May be observed through VT, PT, or MT, More likely harmful to product than a subsurface discontinuity since it provide start for further damage.
Manufacturing Process & Defect.