A consensus mechanism in a private blockchain which essentially gives one entity (or a specific number of entities) with one particular private key the right to make all of the blocks in the blockchain. This is similar to root certificate authorities used in TLS/HTTPS.
Proof of Capacity (POC) is a consensus mechanism used in blockchains that allow the mining devices in the network to use their available hard drive space to decide the mining rights.
Miners still process and validate transactions, but do so by proving that they have ownership of a certain amount of the asset, rather than by performing energy-intensive computations.
A network on the global internet allowed to participate in all available transactions and participate in the consensus protocol to help determine who gets to add blocks on the chain and maintain the shared ledger. E.g., just like a public road has open access to anyone.