"Hot gas" refers to a state of matter in which the particles are moving very quickly and have a high amount of energy. This can result in high temperatures and expansion. Full definition
Unfortunately, at a temperature of just a few million degrees (much cooler than the extremely hot gas in galaxy clusters), it is extremely hard to detect. (sciencemag.org)
Or, it might come from hot gas at the inner boundary of a ring around the planet. (solarviews.com)
The heart of the system to generate groups of photons is a glass cell filled with hot gas vapour. (sciencedaily.com)