A diffuse, hot, magnetized plasma that exists between galaxies in a galaxy cluster and is composed mainly of H, He, and heavy elements. The Intracluster medium (ICM) strongly emits X-rays, making it the most luminous extended X-ray source in Universe. While some of the gas has been stripped out of galaxies, it is also likely that some is also primordial in nature, and has been accreted into the clusters. The origin of the ICM is subject of intense investigation. Broadly, two possibilities have been envisaged. The first one considers the intracluster gas to be once contained in galaxies and later driven in the ICM. This would explain several observations: the presence of high metallicity gas, and H I deficiency of galaxies residing in the cores of rich clusters (which suggests that gas stripping has occurred). Alternatively, the ICM is primordial, originating at the time of cluster formation. Actually the ICM may result from a combination of both scenarios.