A Unique Key is a database constraint that ensures each record in a table is distinct. It prevents duplicate entries by enforcing a rule that no two rows can have the same value in the specified column(s). This is crucial for maintaining data integrity and allows for efficient data retrieval. In relational databases, a unique key can be applied to one or more columns. For example, in a table of customers, the email address could be set as a unique key, ensuring that no two customers can register with the same email. This helps in identifying records easily and accurately.