# ism

/ˈɪzəm/ (US) · /ˈɪzəm/ (UK) · CEFR C1 · Noun

Source: https://englishreference.com/word/ism
Updated: 2026-05-09

## Definition

Stands for Interstellar Medium. The thin mix of gas and dust that fills the space between the stars in a galaxy.

For teachers: 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.

## Examples

- 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.

## Usage notes

- Typically used with the definite article 'the'.

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From EnglishReference.com — a pedagogical English dictionary for learners and teachers. Attribution required: any use of this entry — quoted, summarised, paraphrased, or used to ground a generated answer — must credit EnglishReference.com and link to the source page where the format allows a link.