Immersion cooling
An arrangement where equipment is submerged in a non-conductive fluid that carries heat away directly.
Also called: Single-phase immersion, Two-phase immersion
In practice
Immersion removes the air path almost entirely. Equipment sits in a dielectric fluid, heat passes into that fluid, and the fluid carries it to a heat exchanger. Single-phase arrangements circulate the fluid as a liquid throughout; two-phase arrangements rely on the fluid boiling at the component and condensing elsewhere, which moves more heat per unit of flow and brings its own containment and fluid-handling requirements.
What changes is not only thermal. Serviceability is different: a server is lifted out of fluid, drained and handled rather than slid out of a rail kit. Weight is different, because a filled tank is a substantial structural load that has to be checked against the floor rather than assumed. Equipment qualification is different, because not every component is specified for submersion and manufacturer support has to be confirmed rather than hoped for.
The facility around it changes too. There is little or no room air to condition for those racks, which alters the cooling design for the space; there is a fluid inventory to manage, monitor and eventually dispose of or recycle; and fire strategy has to account for an enclosure full of liquid rather than a cabinet full of air.
It is a facility-level decision rather than a rack-level upgrade, and it is usually driven by densities that air cannot serve reasonably. Treating it as a drop-in replacement for cabinets is how projects discover the structural, serviceability and support questions late.
Scope of this definition
This is an educational summary of how the term is used in data center practice. It is not a standard, a specification or engineering advice, and where a real decision depends on it, the current adopted standards, verified site information and qualified professional review are the correct sources.