What is a free-running oscillator also referred to as?

A free-running oscillator is best referred to as an astable multivibrator. This type of oscillator continuously oscillates between its high and low states without the need for external triggering, thus producing a square wave output.

In astable configuration, the circuit cycles between two unstable states, constantly generating periodic signals, which makes it particularly useful in applications like timers, clock pulses, and tone generation, where a stable oscillation is required. The fact that it does not have a stable state is key to its operation, differentiating it from other types of multivibrators that can be triggered to change state by external inputs.

Compared to other types of multivibrators, such as monostable and bistable, the astable multivibrator is notable for its ability to oscillate freely, making it the most fitting definition for a free-running oscillator. Monostable multivibrators produce a single output pulse triggered by a signal, while bistable multivibrators maintain their state until they are triggered by an external input, neither of which aligns with the concept of a free-running oscillator.