A piece of code generated in asymmetric-key encryption process, paired with a public key, to be used in decrypting information hashed with the public key.
What Is a Private Key/Secret Key?
Some wallets also use general encrypted files accessible through a password, which is simply a level above raw key. However, the most common method is through generation of mnemonic 12-24 phrases using the BIP39 protocol. This makes them easier to remember but hard to guess; e.g “shampoo plane stingray gully mollusk anyway clarify observing echo bazooka heading replica.”
Interestingly, some wallets like the CoolWallet S hardware wallet uses numbers instead of words for its recovery seed and then uses a checksum to verify that it’s been correctly recorded at the end. This is to help non-English speakers avoid spelling mistakes such as “plan” instead of “plane.” Such a small mistake could make it impossible to access your wallet.
Another option is to write them on pieces of paper and keep them inside safes or deposit boxes. The most common way to safeguard them is by using hardware wallets in order for them to be kept offline. There is also a niche category of users that store them on specially designed holders made of titanium and other fire resistant robust materials.