What is the time constant of an RC circuit and what does it represent?

The time constant (tau) of an RC circuit equals R times C (tau = RC), measured in seconds. It represents the time required for the capacitor voltage to reach approximately 63.2% of its final value during charging, or drop to 36.8% during discharging. After 5 time constants, the capacitor is considered fully charged or discharged (99.3% complete).