Prisilla has a reasonable good definition of HPC. I would change only " it has thousands of computer connected over the network." to "it has thousands of computational processors connected over an internal high-speed network" to distinguish HPC from a cluster of workstations.
HPMC normally is an abbreviation for the chemical Hydroxy propyl methyl cellulose. I've not seen its use in the context of computation before.
High-Performance Computing most generally refers to the practice of aggregating computing power in a way that delivers much higher performance than one could get out of a typical desktop computer or workstation in order to solve large problems in science, engineering, or business.
The High-Performance Multi Computing is the aggregation of computing power from multiple heterogeneous computing sources.