cerulean (adj.)

"sky-colored, sky-blue," 1660s, with -an + Latin caeruleus "blue, dark blue, blue-green," perhaps from a dissimilation of caelulum, diminutive of caelum "heaven, sky," which is of uncertain origin (see celestial). The Latin word was applied by Roman authors to the sky, the Mediterranean, and occasionally to leaves or fields. The older adjective in English was ceruleous (1570s). As a noun, from 1756. The artist's cerulean blue is from 1885.

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Definitions of cerulean from WordNet
cerulean (n.)
a light shade of blue;
Synonyms: azure / sapphire / lazuline / sky-blue
cerulean (adj.)
of a deep somewhat purplish blue color similar to that of a clear October sky;
Synonyms: azure / sky-blue / bright blue