You may "refer" to a different relation and have referential integrity between relations. Maybe "joinable" is also acceptable. You might also explicitly mention

You may "refer" to a different relation and have referential integrity between relations. Maybe "joinable" is also acceptable. You might also explicitly mention "foreign key" constraints. Purely mathematically I believe. In relational algebra there is no term, as from a mathematical point you can join any relations. Whether that makes sense is a different question.