Substellar companion is a generic term for an astronomical body orbiting a star.
It can describe a celestial bodies too small to be a star, but too big to be called a planet. Alternatively, it is a substellar object such as exoplanet or brown dwarf that is orbiting a star. Objects as low as 8-23 Jupiter masses have been called a substellar companion.
Objects orbiting a star are often called a planet below 13 Jupiter masses and brown dwarf above that. Companions at that planet-brown dwarf borderline have been called Super-Jupiters, such around the star Kappa Andromedae. Nevertheless, objects as small as 8 Jupiter masses have been called a brown dwarf.
A substellar companion is thought to exist in the binary star system SDSS 1212. Substellar companions have been confirmed by analyzing astrometric data from Hipparcos.