Crystal oscillator
A clock source that uses a vibrating quartz crystal to produce a frequency stable to within parts per million.
Quartz is piezoelectric: apply a voltage and it flexes, flex it and it produces a voltage. Cut to the right shape it resonates at a single frequency with very little drift. Walter Cady built the first crystal-controlled oscillator in 1921, and Bell Labs' quartz clock of 1927 made the pendulum obsolete.
A typical microcontroller board carries two: one at a few MHz that a PLL multiplies up to the system clock, and one at 32,768 Hz — exactly 215 — for the real-time clock, because a binary counter then overflows once a second. The on-chip RC oscillator is fine for plenty of jobs but can be off by a percent or two, which is enough for a UART to start reading garbage. A ±20 ppm crystal drifts less than two seconds a day.