SQL-data consists entirely of table variables, called "base tables".
"Table variables" is a totally stupid and ridiculously silly phrase. A junior DBA would never use mutter such a silly thing. There is no dodging it. It's also not a term. It's not in italics, or quotes. "Base Tables" is a term, at least insofar as it's used elsewhere,
A base table is a site that holds a table value (see Subclause 4.3, “Tables”). All SQL-data is held in base tables.
So I'm not guessing "table variables" is supposed to be a phrase meaning things that hold state and mutate (variable). And, a base table is the lowest form of storing data without any higher abstractions.
The only place I've seen "base table" used is in SQL Server. An important aside is that the SQL spec differentiates between a few types of Base Tables but they're in the weeds in their parlance, an example is a "regular persistent base table" which is just what most people think of when they say "table".
Yes there is a "relation", and you can even argue for "relation variable". Show me a "Table Variable" -- outside of the Microsoft parlance which is in reference to a T-SQL data type that has 0-todo with this context here.