Function dependencies are the consequence of a interrelationship among attribute of an entity represented by a relation. Function dependencies may also be due to the relationship between entities that are also represented by a relation. A relation R, attribute Y of R is a function dependencies. An attribute X of R if and only if each X value on R has associated with it precisely one Y value in R.
Fully Function Dependencies :- when all non-key attribute are depend on the key attribute. It is called full function dependency.