What is the dominant component difference between a Group and a Cluster?

Clusters possess a vast intracluster medium (ICM), Groups do not emphasize it

While both galaxy groups and clusters exist on a continuum of mass, a key differentiator lies in their mass distribution and evolutionary maturity, particularly concerning the hot gas component. Galaxy clusters, being the most massive systems, host vast reservoirs of the intracluster medium (ICM), which is a significant contributor to their total mass budget and is readily observed via X-ray emission. Galaxy groups, being less massive and often less dynamically evolved, may contain some hot gas, but the ICM does not dominate their mass budget or define their structure to the extent seen in true clusters. Clusters are characterized by having substantial contributions from Stars, Dark Matter, and the vast ICM, while groups are generally less dominated by this extensive hot gas component.