Private key

A private key is the secret number that controls a single blockchain account and signs its transactions, so whoever holds it can move everything in that account. ethereum.org's accounts page explains that an account is a pair of cryptographic keys and that the private key is what signs transactions, so it grants custody of the funds: you never really hold cryptocurrency, you hold private keys. An Ethereum private key is 64 hex characters. The public address is derived from it, and the private key cannot be worked out from the public one.

Wallets that manage many accounts give each one its own private key, all generated from one secret recovery phrase, as Ledger explains. So the seed phrase unlocks every account, while a private key unlocks one. The ethereum.org glossary puts the rule in capitals: DO NOT SHARE IT. A site that asks you to paste a private key to claim an airdrop is asking for the account itself.