The Heterodyne "Principle"
A working definition of heterodyning: To generate new frequencies by mixing two or more signals
in a nonlinear device such as a vacuum tube, transistor, or diode mixer.

Reginald Aubrey Fessenden (1866-1932) is usual credited with the invention of the heterodyne principle (reference).

As an example, we revisit our discussion of DSB-AM where we see how two frequencies are shifted by the nonlinear operation of multiplication.
Primary Example of a Nonlinear Characteristic - Diode (Rectifier) Current-Voltage Characteristics