Azure is a blue color on the HSV color wheel (Hue, Saturation, Value) at 210 degrees. Azure is the hue that is halfway between blue and cyan. Its complementary color is orange.
Azure is a near synonym for the color blue. Commonly it refers to a bright blue, resembling the sky on a bright, clear day.
The term azure derives from the Persian لاژورد (lazhward), which was the name of a place in modern Turkestan known for its deposits of lapis lazuli (“stone of lazhward”). The word was adopted into Old French by the twelfth century.

From the French it was adopted into English as a near synonym for "blue". The first recorded use of azure as a color name in English was in 1374.
The use of the term spread through the practice of heraldry, where “azure” represents a blue color in the system of tinctures. In engravings, it is represented as a region of parallel horizontal lines, or by the abbreviation az. or b. In practice, azure has been represented by any number of shades of blue. In later heraldic practice a lighter blue, called bleu celeste (“sky blue”), is sometimes specified.

Azure also describes the color of the mineral azurite, both in its natural form and as a pigment in various paint formulations. In order to preserve its deep color, azurite was ground coarsely. Fine-ground azurite produces a lighter, washed-out color. Traditionally, the pigment was considered unstable in oil paints, and was sometimes isolated from other colors and not mixed. Modern investigation of old paintings, however, shows that the pigment is very stable unless exposed to sulfur fumes.

In some color systems, azure is defined more specifically. In the X11 color system which became a standard for early Web colors, azure is a pale blue color. "United Nations blue", as it's called by the United Nations, is the light blue color of the Flag of the United Nations and of the helmets worn by the United Nations peacekeeping forces.
In Russian, "голубой" (goluboj, azure or cyan) and "синий" (sinij, blue or navy blue) are not two shades of the same color, but distinguished in the way red and pink are distinct colors in English. A similar distinction exists between "azzurro" (azure, but used to indicate various shades of light blue) and "blu" (blue) in Italian.

From Wikipedia