Any star that varies in brightness. There are two broad categories: extrinsic variables, which undergo a real change in luminosity of either an individual star or some element in a binary system. Certain stars may combine both forms of variation. The standard reference for the classification of variable star types and their nomnclature is the General Catalogue of Variable Stars (GCVS). The current total of designated variables is over 30000.

The classification of variable stars was originally based upon the form of their light-curve, its amplitude, and periodicity (or lack of it). Increasingly, however, the physical structure of the stars (and binaries) are used to define six groups of variables, each with specific features. The groups are subdivided into individual types of variation, often named after specific stars. These include:

• Eruptive variables exhibit unpredictable changes in the form of flares or fades, most of which originate in chromospheric or coronal in the form of flares or fades, flare stars, R Coronae Borealis stars, and T Tauri stars.
• Pulsating variables expand and contract or experience wave-like motion of the surface because of fluctuations in the flow of energy from their interiors. Notable examples are Cepheid variables, Mira stars, and ZZ Ceti stars.
• Rotating variables are a small group whose variations arise from non-uniform surface brightness or elliptical shape.
• Cataclysmic variables generally exhibit powerful outbursts with a sudden release of energy. The group includes novae, dwarf novae, and supernovae, and should not be confused with eruptive variables.
• Eclipsing variables are binary stars which exhibit partial or total eclipses of one or both components. They are subdivided, on the basis of the shape of the light-curve, into Algol stars, Beta Lyrae stars, and W Ursae Majoris stars. This group also includes some systems that do not eclipse, but where the distorted shape of the components produce fluctuations in the light-curve.

Optically variable X-ray source bear many similarities to cataclysmic variables (specifically to cataclysmic binaries). The optical variations are often induced by the X-ray variability. Two examples are the AM Herculis stars (or polars) and the HZ Herculis stars (or X-ray pulsars).