Private Key
Quick definition
The secret that proves ownership of your Bitcoin — whoever holds it controls the coins.
A private key is a secret number that authorizes spending from your address. Wallets manage keys for you; the seed phrase is a human-readable backup of them.
Anyone with your private key (or seed phrase) can take your Bitcoin from anywhere in the world. Never type it into websites, never store it in cloud notes, never share it with 'support.'