> Relational databases get their name from the fact that relationships can be defined between tables.
Relational databases get their name from the mathematical concept of a relation, used by the Relational Model, "an approach to managing data using a structure and language consistent with first-order predicate logic, first described in 1969 by English computer scientist Edgar F. Codd, where all data is represented in terms of tuples, grouped into relations." [1][2] (emphasis added)
Recommended Reading: Database In Depth, by Chris Date.