An old word for a new color
The name cyan goes back to the Ancient Greek kyanos, which meant a dark blue enamel or lapis lazuli. Some writers trace it to a dye made from the cornflower instead, whose Latin name is Centaurea cyanus. People first used cyan as a color name in English in 1879. For a long time it was called cyan blue.
Green and blue light at full strength
On a screen, #00FFFF means red is off while green and blue are at their highest value. That makes cyan a secondary color of light. It sits straight across the color wheel from red, so red light plus cyan light gives white. Cyan is one of the X11 color names, and CSS accepts it as a keyword that matches aqua.
Too bright for paper
Printers can't reach this shade. The cyan ink used in four-color printing is darker and less pure, as the CMYK guide explains. The screen version also needs care with text. White text on #00FFFF only reaches 1.25 to 1, while black text gets 16.75 to 1.
Cyan in nature
The planet Uranus looks cyan because methane in its air soaks up red light. Liquid oxygen is a clear cyan liquid. Shallow sea water over sand looks green blue for a similar reason, since water takes in red light. The Wikipedia article on cyan has more examples. For a deeper shade, see dark cyan.