Quadrature AM. A form of amplitude modulation that allows two wireless signals to be transmitted on the same frequency (either DSB-SC style or Standard AM style). This signal takes advantage of the properties of sine and cosine in the frequency domain. As you may be aware, sine and cosine are essentially the same, but 90 degrees out of phase with each other, making them perpendicular. When a Fourier transform is applied to the sine and cosine functions, cosine has only real components while sine has only imaginary components. This makes it possible to separate the signals back apart, since they're out of phase -- though it is more complicated than vanilla Standard AM.
Or, in other symbols, given input signals G1(x) and G2(x), the output signal H(x) is G1(x)*cos(f*t) + G2(x)*sin(f*t), where f is the frequency in radians.
This modulation scheme was what was used for stereo AM, which never caught on.
Compare DSB-SC, Standard AM, and single sideband.