A "relationship" in an ER diagram maps to a "reference" in relational theory. This is part of the type safety/domain system of RDBMSs.
If these concepts are muddled, SQL will never quite make sense :)
Relational database can be expressed in tabular form, but tabular data is not necessarily relational.
> (A "relation" in relational theory is a table with a name, fields with names and types, and the data in the table.)
A relation is a system of one or more functions (in the mathematical sense) each of which has a domain that is a candidate key of the relation and a range that is the composite of the non-key attributes.
From the Wikipedia article on relational databases, subsection relational model. "This model organizes data into one or more tables (or "relations") of columns and rows, with a unique key identifying each row. Rows are also called records or tuples."