Schmitt trigger
An input with two thresholds instead of one, so a noisy signal doesn't chatter between high and low.
An ordinary digital input has a single boundary: above it the signal is a one, below it a zero. A slow or noisy signal hovering near that line gets read as a burst of rapid transitions. A Schmitt trigger switches high only once the voltage crosses an upper threshold, and back low only below a lower one. The gap between them is called hysteresis, and it swallows the noise.
Otto Schmitt invented the circuit in 1934 as a graduate student, while trying to mimic how impulses travel along squid nerve fibres. Today it is built into most microcontrollers' GPIO inputs and into classic chips like the 74HC14. Paired with an RC filter, it is the textbook cure for switch bounce.