A mnemonic phrase is a set of words that helps you remember something important, like a password or a sequence of numbers. It's like a cheat code for your memory. Full definition
For the Ledger Nano S, this means you will be generated a 24 - word mnemonic phrase which is used to create and restore your wallet. (blokt.com)
Generates a new empty wallet based on a unique mnemonic phrase (a list of words from its own dictionary). (howtotoken.com)
The researcher goes on to explain that — once installed — the shady copycat requires users to fill in their private key (or 12 - word mnemonic phrase) to log in. (thenextweb.com)