Multisig
A wallet that needs signatures from more than one key to move money, so that no single key can do it alone.
An ordinary wallet has one private key, and whoever holds it can move everything. A multisig wallet, short for multi-signature, has several keys and a rule, fixed when the wallet is created, about how many of them have to sign. It’s written as “m of n”: in a 2-of-3 wallet there are three keys, and any two of them can move the money.
The point is that no single key is enough. A thief who steals one key has nothing to spend, and someone who loses one key can still move the money with the others. That’s why the keys are kept in different places, and often by different people.
On bitcoin, multisig is part of the network’s own rules. For dollars on networks like Ethereum, it’s a smart contract wallet that checks the signatures before anything moves.
What it adds
More to look after. Each key needs its own backup, and so does the wallet’s configuration: which keys belong to it and how many must sign. On bitcoin, enough keys without that configuration can leave you unable to find the wallet, so it’s backed up too.
What it doesn’t fix
A rule that needs two keys is only as good as the distance between them. If one person or company holds enough keys to sign alone, that’s a single point of failure again. And if the signers approve a transaction without checking what it does, the rule is met and the money still goes. Most of the large thefts from multisig wallets happened one of those two ways.