Astronomy Answers
From the Astronomical Dictionary

The description of the word you requested from the astronomical dictionary is given below.

visible light

Visible light is a form of electromagnetic radiation with wavelengths between about 400 and 800 nm. It seems silly to talk about "visible" light; after all, you can always see light, can't you? Yet, there are good reasons:

1. some kinds of electromagnetic radiation that we cannot directly see are yet called light, for example infrared and ultraviolet "light".
2. the boundary between colors we see well and "colors" we cannot see at all is not sharp but gradual, so we cannot pinpoint an exact border between visible and invisible light.
3. not everybody is equally sensitive to any given color. A color that is just visible to some may be invisible to some others. Should such a color be considered to be light or not?
4. visible light does not differ fundamentally from invisible light (such as infrared and ultraviolet light) that is just beyond the visible colors.

Scientists therefore use the phrase "visible light" for the wavelength range of electromagnetic radiation that the average person can see at least a little bit, with vague boundaries on the low and high sides.