What is a tone (in context of IMD)?

I've seen the term "tone" used a lot in RF literature, but haven't found any helpful explanation of what exactly a tone is. I am looking at tone in context of IMD; i.e. the fundamental input tones. What exactly is a "tone" in this context?

Tone, in this context, typically refers to a sinusoidal signal at a single frequency. For example, if I'm doing a "two-tone" power measurement to check for intermodulation distortion ("IP3 measurements"), then I'll be supplying to my DUT two CW sine waves, one at $$\f_a\$$ and another at $$\f_b\$$. A "one-tone" measurement, by contrast, would be a measurement where I'm supplying only a single sine wave, say, at $$\f_a\$$ only.