Relational short description:
Category of database management systems characterized by the ability to join two data sets by matching on common fields, using mathematics-based relational logic.
RDBMS short description:
Relational Database Management System, a widely used type of database management system characterized by its extensive use of joining as a core operational principle.
I think we should drop the relational tag. That would serve to remove some of the ambiguity between the three tags. Alternatively it would be nice to develop some clear guidance on when the tag relational is more appropriate that the other tags.
*: Click each tag for a list of questions.