An astable multivibrator is an electronic circuit that continuously switches between its two unstable states, producing a square wave output. It typically consists of two resistors and two capacitors connected to a transistor or an operational amplifier. This configuration allows it to generate a periodic signal without requiring any external triggering. The output frequency of an astable multivibrator can be adjusted by changing the values of the resistors and capacitors used in the circuit. It is commonly used in applications such as timers, oscillators, and flashing LED lights, making it a versatile component in electronics.