Explain what is meant by the concept of a unique key in a database?

A unique key in a database is a column or a set of columns whose values uniquely identify each record in a table. It ensures that there are no two rows with the same unique key value in the table. Unlike a primary key, a unique key can contain NULLs (depending on the DBMS), and a table can have multiple unique keys.

Example in SQL:

CREATE TABLE Users (
  id INT PRIMARY KEY,
  email VARCHAR(255) UNIQUE
);

Here, email is a unique key that prevents adding two users with the same email.