One of an important group of yellow giant or supergiant pulsating variables, named after the prototype, Delta Cephei. This general term is commonly applied to more than one stellar type, in particular to the classical Cepheids (sometimes known as Delta Cephei stars), and the less numerous W Virginis stars.

The significance of Cepheid variables became apparent when H. S. Leavitt discovered that their period was directly related to their absolute magnitude. The resulting period–luminosity relationship is used to determine distances. Subsequent work established the existence of two distinct types, with essentially parallel period–luminosity relationships. The classical Cepheids are Population I objects with absolute magnitudes 0.7–2 mag. brighter than the Population II W Virginis stars, as well as larger masses and greater metallicity. Both types undergo radial pulsations in the fundamental mode. At maximum size, Cepheids are typically 7–15% larger than at minimum size.

In the past, other distinct types were regarded as forms of Cepheids, notably those called dwarf Cepheids (AI Velorum and Delta Scuti stars), and short-period Cepheids (RR Lyrae stars). See also beat cepheid; bump cepheid; double-mode variable.