A variable of type "relation", where a "relation" is as defined in the RelationalModel.  Relations can be "base relations" (a set of records), or "views" (relations computed based on other relations/RelationalVariable''''''s via the RelationalAlgebra or RelationalCalculus).

Relations themselves are immutable.  A RelationalVariable, on the other hand, is mutable, and can refer to one relation at one particular instance in time, then another relation at a later time; much as an integer variable can assume different integer values at different moments in time.  (The integer "5" is immutable; a variable "x" that refers to 5 now and 3 later, is not.)

Often called ''relvar'' for short.
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EditHint: merge with RdbRelVar and RelVar