Virial Shock Heating In the review paper on protoclusters, Roderik Overzier (pg 25) mentions the concept of virial shock heating which seems to be a process involved with galaxy cluster formation.

I'm not fully certain what a virialized galaxy cluster is apart from the fact that it's in hydrostatic equilibrium, and is one of the key distinguishing features between a protocluster and a fully formed galaxy cluster.

I vaguely understand that shock heating relates to the heating that occurs when something is travelling at supersonic speed within a medium and produces a shock.
Virial shocks appear when gas falls in toward a galaxy, and the shocked gas stays hot so the pressure in the post-shock gas can support the shock and keep it away from the galactic disk. 
 The alternative is if the post-shock gas cools and loses pressure support, and the (non-virial) shock collapses right down to the galactic disk.  The significance is that if gas falls directly onto the high-density disk, it has different effects than if it meets a low-density shock well away from the galaxy.  There's a lot of information at https://academic.oup.com/mnras/article/345/1/349/984798