Complex sentence: In the relational model, a relation is a (possibly empty) finite set of tuples all having the same finite set of attributes, which is more formally called the sort of the relation, or just referred to as column names.
Split into simpler sentences: 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.