The color cyan first appeared in the English language in the late 1800s, although it had been used in other languages for centuries. It comes from the Greek kyanos (κύανoς), used to refer to dark blue, such as the color of lapis lazuli. It has changed its meaning over time, entering the English language (originally as 'cyan blue') with its original meaning of dark blue, but drifting to mean either a wide range of green-blue (and/or blue-green), or sometimes more specifically a shade of light, bright blue sometimes called aqua or electric cyan.
Those who like technical definitions will sometimes consider true cyan to be the result of equal amounts of green or blue, or alternatively, white light minus red. It is thus the complementary color to red, and so is used as the 'blue' lens in 3D glasses.
In an alternative view, cyan covers the green-blue area of the spectrum in the same way that orange covers the red-yellow area, and covers a wide range of shades from robin egg blue through teal to sea green. The result of these various definitions is that you should never ask for something in cyan until you've actually laid eyes on the color -- there's simply no telling what you might get.
Hex triplet #00FFFF