Page "Tuple" Paragraph 38

In the relational model, a relation is a ( possibly empty ) finite set of tuples all having the same finite set of attributes.

This set of attributes is more formally called the sort of the relation, or more casually referred to as the set of column names.

A tuple is usually implemented as a row in a database table, but see relational algebra for means of deriving tuples not physically represented in a table.