In relational database theory, a functional dependency is a constraint between two sets of attributes in a relation from a database. It specifies the relationship between two sets of attributes where one attribute determines the value of another attribute. A functional dependency is denoted as X → Y, where the attribute set on the left side of the arrow, X, is called the determinant, and Y is called the dependent.

Functional dependencies are used to mathematically express relations among database entities and are very important to understand advanced concepts in Relational Database System. They are used to ensure that the changes made in one attribute do not affect inconsistency in another set of attributes, thus maintaining the consistency of the data in the database.

There are different types of functional dependencies, including trivial functional dependency, non-trivial functional dependency, full functional dependency, partial functional dependency, and transitive functional dependency.

Overall, functional dependency is a fundamental concept in database management systems for ensuring the data consistency and accuracy.