An entrainment defect is a term used in metallurgy

Metallurgy

Metallurgy is a domain of materials science that studies the physical and chemical behavior of metallic elements, their intermetallic compounds, and their mixtures, which are called alloys. It is also the technology of metals: the way in which science is applied to their practical use...

 to describe a defect created in a casting

Casting

In metalworking, casting involves pouring liquid metal into a mold, which contains a hollow cavity of the desired shape, and then allowing it to cool and solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process...

 by the folding-over of the oxidized surface layer of the molten metal

Metal

A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

into the bulk liquid.

Reason

This defect occurs due to the inclusions entrapped inside molten metal when solidified. This is not occurred at the surface of the metal but when tooling is applied on the casting part, underneath this defect can be shown clearly.