What term is used when different encryption keys produce the same ciphertext for identical plaintext?

The concept described in the question refers to a situation where different encryption keys lead to identical ciphertext for the same plaintext. This phenomenon is indicative of a weakness in the encryption scheme, as it implies that the encryption does not sufficiently vary the output for the same input when using different keys.

The correct term associated with this situation is "Clustering." Clustering refers to multiple keys generating the same ciphertext for identical plaintext due to insufficient randomness or entropy in the encryption algorithm. This can potentially allow an attacker to deduce relationships between plaintexts and ciphertexts, undermining the overall security of the encryption system.

In contrast, terms like diffusion and confusion refer to desirable properties of encryption algorithms aimed at enhancing security. Diffusion ensures that a small change in the plaintext or key leads to significant changes in the ciphertext, while confusion aims to make the relationship between plaintext and ciphertext complex and obscure. Complete Avalanche describes a situation where a slight change in input results in a significant change in output, further supporting the overall security goals of encryption systems.