Power-on reset
The circuit that holds a processor in reset at power-up until the supply voltage and clock have settled.
Supply rails don't reach 3.3 V instantly, and a crystal oscillator needs milliseconds to start swinging. A CPU that starts executing before then fetches garbage from a half-awake flash. Power-on reset (POR) holds reset active until the voltage crosses a threshold, then waits a little longer for good measure. Only then is the reset vector fetched.
On older designs such as 8051 and Z80 boards, POR was a resistor and a capacitor on the reset pin. That works as long as the supply ramps up quickly; a sluggish power supply could release reset too early. Modern microcontrollers build POR in, usually next to the brown-out detector, along with a status register recording whether the last restart came from power-up, brown-out or the watchdog — the first thing to check when a device keeps rebooting itself.