Some definitions I found
Short for "magnetic particle inspection". A procedure for checking all steel parts -including suspension pieces, connecting rods, cylinder heads, etc. - for cracks and other defects utilizing a solution of metal particles and fluorescent dye and a black light. Surface cracks will appear as red lines.
A trade name for the equipment and processes used for detecting cracks and other surface discontinuities in iron or steel. A magnetic field is set up in the part to be inspected. and a powder or paste of magnetic particles is applied. The particles arrange themselves around discontinuities in the metal, revealing defects.
A special chemical process, used to check parts for cracks.
But I've never heard of it