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1. The hue of that portion of the visible spectrum lying between green and indigo, evoked in the human observer by radiant energy with wavelengths of approximately 420 to 490 nanometers; any of a group of colors that may vary in lightness and saturation, whose hue is that of a clear daytime sky; one of the additive or light primaries; one of the psychological primary hues.
a. A pigment or dye imparting this hue.
b. Bluing.
a. An object having this hue.
b. Dress or clothing of this hue: The ushers wore blue.
a. A person who wears a blue uniform.
b. blues A dress blue uniform, especially that of the US Army.
5. often Blue
a. A member of the Union Army in the Civil War.
b. The Union Army.
6. A bluefish.
7. Any of various small blue butterflies of the family Lycaenidae.
a. The sky.
b. The sea.
adj. blu·er, blu·est
1. Of the color blue.
2. Bluish or having parts that are blue or bluish, as the blue spruce and the blue whale.
3. Having a gray or purplish color, as from cold or contusion.
4. Wearing blue.
5. Being a trail, as for skiing, marked with a sign having a blue square, indicating an intermediate level of difficulty.
6. Relating to or being a blue state.
a. Gloomy; depressed. See Synonyms at depressed.
b. Dismal; dreary: a blue day.
8. Puritanical; strict.
9. Aristocratic; patrician.
10. Indecent; risqué: a blue joke; a blue movie.
tr. & intr.v. blued, blu·ing, blues
To make or become blue.
blue in the face
At the point of extreme exasperation: I argued with them until I was blue in the face.
into the blue
At a far distance; into the unknown: spontaneously take a trip into the blue.
out of the blue
1. From an unexpected or unforeseen source: criticism that came out of the blue.
2. At a completely unexpected time: a long-unseen friend who appeared out of the blue.

[Middle English blue, bleu, from Old French bleu, of Germanic origin; see bhel- in Indo-European roots.]

blue′ly adv.
blue′ness n.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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White mists were hovering in the silent hollows and violet stars were shining bluely on the brooklands.
The fire was now low, the flames burning bluely and petulantly, with occasional flashes, projecting spectral shadows on the walls--shadows that moved mysteriously about, now dividing, now uniting.
The delegation then visited Bluely Company to review their practices in smart car-sharing and smart rental of electric vehicles in the city, as well as the challenges in the operation and maintenance of vehicles.