Loosely the term is sometimes applied to all slips. More specifically it is used for mixtures of clay and frit (and colour too sometimes) which are painted in a slip state onto clay. Engobes can be more readily built up to opaque thickness than slips. Found on https://ashbrook-ceramics.co.uk/pottery-glossary/
A type slip or underglaze. It is a type of slip that when fired is vitreous or in maturation is halfway between a glaze and a clay. It may be white or colored, and is usually applied to high firing stonewares and porcelains at the green or bisque state. Found on https://www.bathpotters.co.uk/helpful-guides/94-pottery-glossary