The dependency of an attribute on a set of attributes is known as trivial functional dependency if the set of attributes includes that attribute.
Symbolically: A ->B is trivial functional dependency if B is a subset of A.
The following dependencies are also trivial: A->A & B->B
For example: Consider a table with two columns Student_id and Student_Name.
{Student_Id, Student_Name} -> Student_Id is a trivial functional dependency as Student_Id is a subset of {Student_Id, Student_Name}. That makes sense because if we know the values of Student_Id and Student_Name then the value of Student_Id can be uniquely determined.
Also, Student_Id -> Student_Id & Student_Name -> Student_Name are trivial dependencies too.