A 13 or 25 word phrase used to backup a monero account, available in a number of languages. This 25-word phrase (13 words in the case of MyMonero) has all the information needed to view and spend funds from a Monero account.
In the official wallet, the mnemonic seed comprises 25 words with the last word being used as a checksum. Those words correspond to a 256-bit integer, which is the account's private spend key. The private view key is derived by hashing the private spend key with Keccak-256, producing a second 256-bit integer. The corresponding public keys are then derived from the private keys.