A key-value pair is a fundamental data structure used in programming and databases to store information. It consists of two linked elements: a "key," which is a unique identifier, and a "value," which is the data associated with that key. For example, in a dictionary, the word is the key, and its definition is the value. Key-value pairs are commonly used in various applications, including NoSQL databases like Redis and MongoDB, where they allow for efficient data retrieval. This structure simplifies data management, making it easy to access, update, or delete information based on its key.