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.
2.
a. A pigment or dye imparting this hue.
b. Bluing.
3.
a. An object having this hue.
b. Dress or clothing of this hue: The ushers wore blue.
4.
a. A person who wears a blue uniform.
b. blues A dress blue uniform, especially that of the U.S. Army.
5. often Blue
a. A member of the Union Army in the Civil War.
b. The Union Army.
6. A bluefish.
7. A small blue butterfly of the family Lycaenidae.
8.
a. The sky.
b. The sea.