Final answer:
An Initialization Vector (IV) is a 24-bit value that changes with each encryption of a packet and is combined with a shared secret key to enhance security.
Step-by-step explanation:
The 24-bit value that changes each time a packet is encrypted and is then combined with a shared secret key is known as an Initialization Vector (IV). This is used in various encryption algorithms to ensure that the same plaintext will not result in the same ciphertext when encrypted multiple times, thus providing better security. This concept is particularly important in the field of cryptography.