Prompt emission is the immediate release of particles and radiation directly resulting from a nuclear reaction, such as fission or fusion, occurring within picoseconds or nanoseconds of the event. As a research concept in nuclear physics and engineering, it investigates the characteristics, including type, energy spectrum, angular distribution, and precise timing, of these prompt particles (e.g., prompt neutrons and prompt gamma rays) to elucidate reaction mechanisms, quantify energy partitioning, and model the transient behavior of nuclear systems relevant to reactor dynamics, nuclear security, and astrophysical processes.