The intergalactic medium (IGM) is matter such as dust and gas between galaxies. Similar matter between stars within a galaxy is called the interstellar medium. Between galaxies it is often plasma in filaments, mostly ionized hydrogen.
The IGM can heat to 105 to 107 K, called the warm-hot intergalactic medium (WHIM), observed in ultraviolet and low-energy X-ray radiation.
The intracluster medium has a similar meaning, but more specifically means the medium within a galaxy cluster.