disky

Disky is an adjective describing something that has the shape or characteristics of a disk. The term is derived from disk, with the suffix -y to indicate resemblance. In general usage, disky can refer to objects (or features) that are flat and round, such as disk-shaped bodies or structures.

In astronomy, disky has a specialized meaning. It describes galaxies whose isophotes—the contours of equal brightness—exhibit disk-like deviations from a perfect ellipse. These disky shapes often indicate an embedded stellar disk and enhanced rotational support. The complementary category is “boxy” isophotes, which show box-like distortions and are typically linked with slower rotation. Disky ellipticals and S0 galaxies commonly have disky isophotes, and the degree of disky structure is related to the galaxy’s kinematics and formation history. The strength of diskiness is frequently quantified by the a4 Fourier coefficient of the isophotes: positive a4 indicates disky deviations, negative a4 indicates boxy deviations.

Diskiness is a descriptive term rather than a discrete classification, and its interpretation depends on observational data quality and modeling. It is used mainly by extragalactic astronomers studying galaxy structure and evolution. In broader science, disky can also simply describe disk-shaped features or components in non-galactic contexts, reflecting the general idea of disk-like geometry.