Glossary
Private Key
The secret number that lets the holder sign transactions for a given address. Anyone with the private key controls the funds — no exceptions.
A private key is a large random number — 256 bits on most chains — used to produce digital signatures. The public key (and the address derived from it) can be shared freely; the private key cannot.
There is no password reset on a blockchain. If you lose the key, the funds are gone; if someone else gets a copy, they can move the funds and you cannot undo it. Hardware wallets, multisigs, and threshold signatures all exist to reduce the chance of either outcome.