[takeda]

The adjective relational in a relational database comes from mathematical relations, tuples i.e. data in tables.

It's common misconception that it is from foreign keys.

[nailer]

That's interesting - it seems to both be backed by and conflict with a lot of https://en.wikipedia.org/wiki/Relational_model but maybe that's wrong.

I'd still avoid the word 'relational' though - obvious many people will assume 'relational' is related to DB relations rather than tuples (assuming you're right about 'relations' meaning tuples, a lot of the wikipedia contributors are included).