Security that a blockchain inherits from an external source. This security can come in the forms such as preventing invalid state transitions or re-org attacks. Since security of the external blockchain can be shared, multiple blockchains can derive benefits from harnessing its security.
For example, Celestia will provide shared security to blockchains that deploy on it because they will inherit security from the consensus and data availability Celestia provides, which is shared among all chains that will utilize it.
Interchain security is another example of shared security where security isn’t inherited from the blockchain itself. Rather, the Cosmos Hub validator set will be able to opt-in to become a validator for other zones.