A shared lock is a type of database lock that allows multiple transactions to read a resource simultaneously while preventing any of them from modifying it. This ensures that the data remains consistent and unaltered during the read operations. When a transaction holds a shared lock on a resource, other transactions can also acquire shared locks on the same resource, but they cannot obtain exclusive locks until all shared locks are released. Shared locks are commonly used in database management systems like SQL Server and Oracle to maintain data integrity. They are essential in scenarios where many users need to access the same data for reading, such as in reporting or data analysis, without risking data corruption from concurrent writes.