A multi-signature address is a Bitcoin address that is associated with more than one private key. A multi-signature transaction requires more than one private key to authorize the transaction. These are usually set up as m-of-n addresses, which means m keys out of a total of n keys are required to sign a transaction in order for it to be added to the blockchain. As an example, a 2-of-2 multisig address is associated with 2 keys and requires 2 valid signatures in order for the transaction to be valid. A 2-of-3 multisig address is associated with 3 keys and requires 2 valid signatures out of a possible 3 to create a valid transaction. Multisig addresses are harder to steal from than traditional addresses and provide an additional layer of security. They are also useful in several other advanced use cases around fund custody and liability.