ism
n. uncountablen. Stands for Interstellar Medium. The thin mix of gas and dust that fills the space between the stars in a galaxy.
n. Abbreviation for Interstellar Medium. The matter and radiation that exist in the space between the star systems within a galaxy, consisting primarily of hydrogen gas and microscopic dust particles.
New stars form from the gas in the ISM.
Astronomers use radio telescopes to map the density of the ISM across the Milky Way.
The chemical composition of the ISM is constantly evolving as dying stars eject heavy elements back into the surrounding space, enriching the material for future stellar generations.
From the suffix -ism (“belief”), particularly (in the 19th century) in the sense of "social movement". Compare phobia, from -phobia, sophy, from -sophy, itis, from -itis, and ana, from -ana.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.
Typically used with the definite article 'the'.
Last updated May 9, 2026