Use the adjective ultraviolet to describe light with a wavelength that's less than visible light, but longer than x-rays. You can't see ultraviolet light.
You might hear people talk about ultraviolet light as UV radiation — it's the part of sunlight that can give you a sunburn if you're exposed to it for too long. In physics, this kind of electromagnetic radiation is defined by how long its waves are. The word itself, ultraviolet, refers to the fact that its wavelength is shorter than the violet end of the spectrum of visible light. The Latin prefix ultra means "beyond."