Nonce
A nonce is a value that must not repeat within the scope defined by a cryptographic protocol, commonly under one key.
Definition
A nonce is a value used once within a defined cryptographic context, often once per encryption under a particular key. It separates otherwise identical operations so they do not reuse the same internal state or keystream.
Unique does not always mean random
A nonce may be a counter, a randomly generated value with negligible collision risk, or a structured sequence. Some schemes require only uniqueness; others also require unpredictability. The algorithm or protocol defines the rule.
Nonce, IV, salt, and key
A nonce is normally public and is not a secret key. An initialization vector initializes a mode and may have stricter unpredictability requirements; in some schemes the IV is the nonce. A salt separates password derivations and may be reused only according to different rules.
The effect depends on the scheme, but it may reveal plaintext relationships, enable forgery, or completely destroy confidentiality and integrity.
Not always. A counter can be ideal when uniqueness is guaranteed. Follow the exact requirements of the algorithm.
It is commonly stored or sent alongside the ciphertext because the recipient needs it and it usually need not be secret.