Brown dwarfs are substellar objects not massive enough to sustain hydrogen-1 fusion reactions in their cores, unlike main-sequence stars. They occupy the mass range between the heaviest gas giants and the lightest stars, with an upper limit around 75 to 80 Jupiter masses (M_J). Brown dwarfs heavier than about 13 M_J are thought to fuse deuterium and those above ~65 M_J, fuse lithium as well. Brown dwarfs may be fully convective, with no layers or chemical differentiation by depth.

The defining differences between a very low-mass brown dwarf and a gas giant (~13 M_J) are debated. One school of thought is based on formation; the other, on the physics of the interior. Part of the debate concerns whether "brown dwarfs" must, by definition, have experienced fusion at some point in their history.

Stars are categorized by spectral class, with brown dwarfs being designated as types M, L, T, and Y. Despite their name, brown dwarfs are of different colors. Many brown dwarfs would likely appear magenta to the human eye, or possibly orange/red. Brown dwarfs are not very luminous at visible wavelengths.