Seed phrase
Twelve or twenty-four words that are the entire wallet — whoever has the words has the money, regardless of who bought it.
The phrase is a human-readable way of writing down a random number. The BIP-39 standard uses a 2048-word list where each word carries eleven bits, and from that number all the wallet's key pairs are derived deterministically. One backup therefore covers an unlimited number of addresses, and the same phrase can restore the wallet in any compatible software.
Being that portable also makes it the obvious target. No legitimate service ever asks for the phrase, which is precisely what every scam attempt does. Photographing it or saving it in the cloud converts an offline protection into an online problem — the recommendation is paper or metal in physical safekeeping.