A mnemonic seed, also known as a seed phrase or recovery phrase, is a set of words used to generate a deterministic wallet. It acts as a backup for the private keys of cryptocurrency wallets. Mnemonic seeds are typically composed of 12, 18, or 24 words in a specific order that can be used to recover access to the wallet in case the original wallet is lost, damaged, or stolen. It is crucial to keep mnemonic seeds private and secure as anyone with access to them can potentially access the associated funds.