The formal definition of a database schema is a set of formulas (sentences) called integrity constraints imposed on a database.
These integrity constraints ensure compatibility between parts of the schema.
The term n-tuple refers to a tuple of degree n (n ≥ 0). The term relation schema refers to a heading paired with a set of constraints defined in terms of that heading.
A relation can thus be seen as an instantiation of a relation schema if it has the heading of that schema and it satisfies the applicable constraints.
A set of attributes in which no two distinct elements have the same name is called a heading.
A set of tuples having the same heading is called a body.
A relation is thus a heading paired with a body, the heading of the relation being also the heading of each tuple in its body.