What is a tone (in context of IMD)?
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.