Usually pronounces ROY-G-BIV. An acronym used to remind people of the order of wavelength size in the visible light spectrum of electromagnetic waves. In order it reads Red Orange Yellow Green Blue Indigo Violet. Also this is the seven colors that stand out most in any true prism or rainbow light effect. For those that want numbers red has the highest wavelength at around 650 nm and the wavelengths go steadily down till you get to violet that has a wavelength of around 400 nm. Here's a nice little picture pointing to where red is to give you and idea of the spectrum[Courtesy of NASA]:
See also this thread: http://www.controlbooth.com/forums/lighting/13483-roygbiv-through-rosco-gels.html.