FM synthesis
Building timbres by letting one oscillator wobble another's frequency at audio rate, so two plain sine waves produce a dense spectrum of overtones.
John Chowning at Stanford noticed in 1967 that vibrato pushed fast enough stops sounding like vibrato and starts generating new sidebands around the fundamental. The university patented the method and licensed it to Yamaha in 1975, which shipped it in the DX7 in 1983 — the first digital synth to sell in huge numbers, and a patent that became one of Stanford's best earners.
Each sine generator is called an operator, and an algorithm decides who modulates whom. The ratio between frequencies decides whether the result is harmonic like an electric piano or clangorous like a bell; modulation depth decides how bright it gets. Powerful and cheap in silicon, but notoriously hard to program by hand — most DX7 owners stuck to the factory presets, which is why the 1980s sound the way they do.