A shared lock is a type of database lock that allows multiple transactions to read a resource simultaneously while preventing any transaction from modifying it. This ensures that the data remains consistent and accurate during the read operations. When a transaction holds a shared lock on a resource, other transactions can also acquire shared locks, but exclusive locks are not permitted. Shared locks are commonly used in database management systems like SQL Server and Oracle to facilitate concurrent access to data. They are essential for maintaining data integrity, especially in environments where many users need to access the same information without interference.