Other forms: skies
The sky is everything you see when you look straight up outside. A blue sky, the night sky, clear skies.
Its blue comes from short wavelengths scattering more in the air, which is also why sunsets are red — the blue has been scattered out of the long path. The plural skies is used for weather: clear skies, overcast skies. Note the sky's the limit; out of a clear blue sky; pie in the sky; and that sky originally meant cloud, which is why sky-blue took so long to be a phrase.
1nounskyEverything you see when you look straight up outside.
“The sky was clear and full of stars.”
sky is the space above the earth.
Old Norse sky meant a cloud, and English borrowed it in that sense. It displaced Old English heofon for the physical sky, leaving heaven to the religious meaning.
Proto-Indo-European
*(s)kewH- — to cover; to conceal, hide
Proto-Germanic
*skiwją — cloud; sky
Old Norse
ský — cloud
Middle English
sky — sky; cloud; mist
English, today
sky
Built from
The noun is derived from Middle English sky (“sky; cloud; mist”), also spelled ski, skie, schi, schie, schye, sci, scki, scky, scy, skei, skiʒe, skiȝe, sky, skye, (Early Middle English) skige, from Old Norse ský (“cloud”), from Proto-Germanic skiwją (“cloud; sky”), from skiwô (“cloud; cloud cover, haze; sky”) (whence Old English sċēo (“cloud”) and Middle English skew (“air; sky; (rare) cloud”)), from Proto-Indo-European (s)kewH- (“to cover; to conceal, hide”). Partly displaced heofon, which survives in the reflex heaven, still sometimes used in the sense of sky, but usually in high or poetic register. The verb is derived from the noun.
Etymology from Wiktionary, CC BY-SA