This symbol represent the impulse response, the output signal of a system when exposed to an impulse.

This symbol represent the impulse response, the output signal of a system when exposed to an impulse. This encompasses both finite (FIR) and infinite (IIR) responses, characterizing the system's reaction to a specific, typically singular, input. The unit impulse response is a special case where the input is zero at all times except for one instant when it equals one.