A celestial object in which high-energy processes such as accretion on to a compact object, the shock wave from a supernova, a stellar wind, or the hot gas in stellar coronae give rise to detectable X-ray emission. The first X-ray sources to be discovered were mostly objects such as X-ray binaries. Subsequently, supernova remnants, active galactic nuclei, and hot white dwarfs were also found to be X-ray sources. Higher-sensitivity missions have revealed that most objects are X-ray sources at some level. Currently, around 500 000 X-ray sources are known, the majority discovered by XMM-Newton.