The cryptographic key kept secret and used to sign transactions is called a private key.
In public-key cryptography, a private key is mathematically linked to a corresponding public key. The private key proves control of an account or address by producing a digital signature, while anyone can use the public key to verify that signature. On a blockchain, this lets a network confirm that a transaction was authorized without revealing the secret key.
A private key is not the same as a wallet address. An address is a publicly shareable destination for receiving funds, whereas the private key authorizes spending. It is also different from a seed phrase: a seed phrase is a human-readable backup that can generate one or more private keys, depending on the wallet’s design.
Because transactions signed with a private key generally cannot be reversed by the network, losing or exposing the key can permanently lock or endanger the associated assets.