The Planck constant is a fundamental physical constant that relates the energy of a photon to its frequency. It is denoted by the symbol h and has a value of approximately 6.626 \times 10^-34 joule-seconds. This constant is crucial in the field of quantum mechanics, as it helps explain the behavior of particles at very small scales. Named after the physicist Max Planck, who introduced it in 1900, the Planck constant plays a key role in the quantum theory. It signifies the quantization of energy levels, meaning that energy is not continuous but comes in discrete packets called quanta. This concept revolutionized our understanding of atomic and subatomic processes.