Final answer:
A number that is used only one time and then discarded in cryptography is called a Nonce. It is a random or pseudo-random number used to prevent replay attacks and to provide variations in hashing functions.
Step-by-step explanation:
A number that is used only one time then discarded is called a Nonce. In cryptography, a nonce is an arbitrary number that can only be used just once. It is often a random or pseudo-random number issued in an authentication protocol to ensure that old communications cannot be reused in replay attacks. They are also used in hashing functions, such as in the creation of a cryptographic hash, where they serve to provide variations in the input to a hash function, with the intent of ensuring that the same input does not produce the same output hash.
Options like Chain, Salt, and IV have their own meanings in cryptography: a Chain refers to a sequence that links blocks in blockchain technology; a Salt is a random data that is used as an additional input to a one-way function that hashes data, passwords, or passphrases; and an IV or Initialization Vector is a random or pseudo-random number used to ensure that the same plaintext will encrypt to different ciphertext in a cipher block chaining.