RC time constant (τ)
τ = R × C: the time for a capacitor charging through a resistor to cover ~63% of the remaining distance to its target. Five τ is 'fully' charged (99.3%) — the universal clock of analog electronics.
In practice
Everything timing-flavored reduces to it: debounce delays, filter cutoffs, 555 periods, soft-start ramps, I²C rise times. If a circuit waits, charges, settles, or filters, find the R and the C and you've found its schedule.