Multi-sig
A wallet that requires multiple signatures to execute a tx.
Regular wallet: 1 private key = full control. Multi-sig: you set e.g. 3-of-5. To send funds you need signatures from at least 3 different wallets. Standard for DAO treasuries, crypto companies and people with large sums. Most popular: Safe (formerly Gnosis Safe). Pros: one compromised key isn't enough to steal. Cons: more complex operations, signers must coordinate.
In plain words
A regular wallet has one key, whoever has it takes everything. Multi-sig requires several signatures (e.g. 3-of-5) to send a transaction. Ideal for large sums or company accounts: one stolen key isn't enough for a thief. Most popular implementation: Safe (formerly Gnosis Safe).
Example
The Uniswap DAO treasury ($6B+) is a 6-of-10 multi-sig. 6 signatures required for any payout.
Also known as: safe, gnosis safe