A permissioned blockchain is a type of blockchain where the ability to interact with the network, such as writing and reading data, is only accessible to approved actors.
Permissioned blockchains centrally define the rights that a node has. Since nodes are generally trusted entities, there is significantly less risk of a 51% attack. Further, software updates are more efficient to coordinate since node operators are usually subject to a uniform set of established governance procedures. This approach contrasts permissionless blockchains such as Bitcoin or Ethereum, where any entity can run a node and participate in consensus.