If a celestial body lacks the necessary mass for fusion, what might it be classified as instead of a true star?

Answer

A planet or a brown dwarf

A body that does not have sufficient mass to create the pressure and temperature needed to force sustained nuclear fusion remains classified as a planet or, if it is massive enough to start but fail sustained fusion, a brown dwarf, which is described as a 'failed star'.

If a celestial body lacks the necessary mass for fusion, what might it be classified as instead of a true star?
sunastronomystarcelestial body