Private Key
In plain English
The secret number that proves you own an address and authorises spending from it. Whoever holds the private key holds the coins.
What is a private key?
A private key is a very large random number that controls a blockchain address. It is used to create digital signatures — the cryptographic proof that a transaction was authorised by the owner of the funds. Without it, no one can move coins from that address; with it, anyone can.
How keys and addresses relate
From a private key, the wallet derives a public key, and from that a public address. The maths works in one direction only: you can compute the address from the key in an instant, but you cannot work backwards from the address to the key. That asymmetry is what makes it safe to publish your address.
When you send a transaction, your wallet signs it with the private key. Nodes verify the signature against your public key and accept the transaction — without the key ever leaving your device.
Custody: who actually holds it
If you use a self-custody wallet, you hold the key. If you keep crypto on an exchange, the exchange holds it and your balance is simply an entry in their database. Hence the saying "not your keys, not your coins": funds on an exchange depend on that company staying solvent and honest.
Practical implications
Keys cannot be reset. There is no password recovery, no support line and no authority that can reissue them. Losing the key — and its seed phrase backup — means the coins stay at that address forever, visible to everyone and spendable by no one.
In everyday use you rarely handle a raw private key. Wallets manage keys for you and back them up through the seed phrase, which is why protecting that phrase is the practical version of protecting your keys.