A person or entity that has partial control and access over a cryptocurrency wallet.
Multi-signature wallets are a more secure variant of a cryptocurrency wallet: they require multiple people or organizations to provide their own unique keys before allowing access to the stored funds. The signature requirements can be configured in various ways: for example, 2-of-3, one of the common ones, needs two out of a total of three keys to unlock funds. The people using their private keys, in this context, are co-signers.
Co-signing the release of cryptocurrency funds may be required in different scenarios. One of the most popular ones is simple security enhancement of a personal wallet: by using a multi-signature wallet and storing the signatures needed to open it on several different devices, a single user can protect themselves from the loss or theft of one or more of the keys.
Another important use case is relevant to businesses: storing the majority of a company’s cryptocurrency funds on a single-signature wallet subjects them to excessive security and organizational risks. By requiring the keys of a majority of multiple co-signers (for example, board members) a multi-sig wallet protects its contents from misuse or outright theft by a single employee.