A term now generally used to describe stars which are readily detectable at centimetre wavelengths. Causes of this emission include extreme surface magnetic activity, as in young red dwarfs; membership of a closely interacting binary such as Algol; or the existence of a hot, ionized wind that produces emission from plasma, as in the Be star P Cygni. The term was originally used for cosmic sources of radio waves which appeared very bright and compact within the context of early radio interferometers. It is now known that the vast majority of these original ‘radio stars’ are active galactic nuclei.