  Galaxy clusters are the largest and most massive bound objects resulting from
cosmic hierarchical structure formation. Baryons account for somewhat more than
10% of that mass, with roughly 90% of the baryonic matter distributed
throughout the clusters as hot ($T>1$ keV), high-$\beta$, very weakly
collisional plasma; the so-called "intracluster medium" (ICM). Cluster mergers,
close gravitational encounters and accretion, along with violent feedback from
galaxies and relativistic jets from active galactic nuclei, drive winds,
gravity waves, turbulence and shocks within the ICM. Those dynamics, in turn,
generate cluster-scale magnetic fields and accelerate and mediate the transport
of high-energy charged particles. Kinetic-scale, collective plasma processes
define the basic character and fundamental signatures of these ICM phenomena,
which are observed primarily by X-ray and radio astronomers.
