The etymology of Astronism derives from the prefix Astro- relating to celestial bodies (such as stars, planets, galaxies, and comets etc.) and is ended with the suffix -ism which pertains to a system of belief or a cultural or ideological movement.

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Therefore, 'Astronism' literally means a system of belief or cultural movement based on astronomy, or celestial bodies which is reflected in the theme of the religion/organised philosophy. The history of the use of the term Astronism holds its roots during The Year of Prolifics, the five years before which the philosophy being developed by Cometan was referred to Millettism as derived from Millettarian culture.

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Astronism was settled on as the trade appellation for the religion due to its strong associations with an astronomical theme, its simplicity in spelling, and due to the issues of inaptronymity associated with the use of the Millettism. Another reason was that Astronist culture and philosophy during the time of The Year of Completion was beginning to distinguish itself from the wider Millettic culture from which it originated.

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Despite this, Millettism is still the official name for the religion and is used in the title of the religion's proprietor organisation, The Institution of The Philosophy of Millettism. An additional reason for the change was that Cometan did not want people to immediately associate Millettism as being a religion idolising the fictional character of Jesse Millette.