Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram, with absolute visual magnitudes between about −3 and −8. The temperatures of supergiant stars range from about 3,400 K to over 20,000 K.

The title supergiant, as applied to a star, does not have a single concrete definition. The term giant star was first coined by Hertzsprung when it became apparent that the majority of stars fell into two distinct regions of the Hertzsprung–Russell diagram. One region contained larger and more luminous stars of spectral types A to M, which received the name giant. Subsequently, as they lacked any measurable parallax, it became apparent that some of these stars were significantly larger and more luminous than the bulk, and the term super-giant arose, quickly adopted as supergiant.

Supergiants with spectral classes of O to A are typically referred to as blue supergiants, supergiants with spectral classes F and G are referred to as yellow supergiants, while those of spectral classes K to M are red supergiants. Another convention uses temperature: Supergiants with effective temperatures below 4800 K are deemed red supergiants; those with temperatures between 4800 and 7500 K are yellow supergiants, and those with temperatures exceeding 7500 K are blue supergiants. These correspond approximately to spectral types M and K for red supergiants, G, F, and late A for yellow supergiants, and early A, B, and O for blue supergiants.

Supergiant stars can be identified on the basis of their spectra, with distinctive lines sensitive to high luminosity and low surface gravity. In 1897, Antonia C. Maury had divided stars based on the widths of their spectral lines, with her class "c" identifying stars with the narrowest lines. Although it was not known at the time, these were the most luminous stars. In 1943, Morgan and Keenan formalised the definition of spectral luminosity classes, with class I referring to supergiant stars. The same system of MK luminosity classes is still used today, with refinements based on the increased resolution of modern spectra. Supergiants occur in every spectral class, from young blue class O supergiants to highly evolved red class M supergiants. Because they are enlarged compared with main-sequence and giant stars of the same spectral type, they have lower surface gravities, and changes can be observed in their line profiles. Supergiants are also evolved stars with higher levels of heavy elements than main-sequence stars. This is the basis of the MK luminosity system, which assigns stars to luminosity classes purely from observations of their spectra.

In addition to the line changes due to low surface gravity and fusion products, the most luminous stars have high mass-loss rates and resulting clouds of expelled circumstellar materials, which can produce emission lines, P Cygni profiles, or forbidden lines. The MK system assigns stars to luminosity classes: Ib for supergiants; Ia for luminous supergiants; and 0 (zero) or Ia⁺ for hypergiants. In reality there is much more of a continuum than well-defined bands for these classifications, and classifications such as Iab are used for intermediate-luminosity supergiants. Supergiant spectra are frequently annotated to indicate spectral peculiarities, for example B2 Iae or F5 Ipec.

Supergiants can also be defined by a specific phase in the evolutionary history of certain stars. Stars with initial masses above 8-10 M_☉ quickly and smoothly initiate helium-core fusion after they have exhausted their hydrogen, and continue fusing heavier elements after helium exhaustion until they develop an iron core, at which point the core collapses to produce a Type II supernova. Once these massive stars leave the main sequence, their atmospheres inflate, and they are described as supergiants. Stars initially under 10 M_☉ will never form an iron core and in evolutionary terms do not become supergiants, although they can reach luminosities thousands of times the Sun's. They cannot fuse carbon and heavier elements after the helium is exhausted, so they eventually just lose their outer layers, leaving the core of a white dwarf. The phase where these stars have both hydrogen- and helium-burning shells is referred to as the asymptotic giant branch (AGB), as stars gradually become more and more luminous class M stars. Stars of 8-10 M_☉ may fuse sufficient carbon on the AGB to produce an oxygen-neon core and an electron-capture supernova, but astrophysicists categorise these as super-AGB stars rather than supergiants.

There are several categories of evolved stars that are not supergiants in evolutionary terms but may show supergiant spectral features or have luminosities comparable to supergiants.

Asymptotic-giant-branch (AGB) and post-AGB stars are highly evolved lower-mass red giants with luminosities that can be comparable to more massive red supergiants, but because of their low mass, their being in a different stage of development (helium shell burning), and their lives ending in a different way (planetary nebula and white dwarf rather than supernova), astrophysicists prefer to keep them separate. The dividing line becomes blurred at around 7–10 M_☉ (or as high as 12 M_☉ in some models), where stars start to undergo limited fusion of elements heavier than helium. Specialists studying these stars often refer to them as super AGB stars, since they have many properties in common with AGB, such as thermal pulsing. Others describe them as low-mass supergiants since they start to burn elements heavier than helium and can explode as supernovae. Many post-AGB stars receive spectral types with supergiant luminosity classes. For example, RV Tauri has an Ia (bright supergiant) luminosity class despite being less massive than the Sun. Some AGB stars also receive a supergiant luminosity class, most notably W Virginis variables such as W Virginis itself, stars that are executing a blue loop triggered by thermal pulsing. A very small number of Mira variables and other late AGB stars have supergiant luminosity classes, for example α Herculis.