Although it sounds contradictory, a dimensionless-quantity is a quantity whose dimension is the identity-dimension. All numeric tensors, including real numbers, are nondimensional quantities.
(<=> (Dimensionless-Quantity ?X) (And (Constant-Quantity ?X) (= (Quantity.Dimension ?X) Identity-Dimension)))
(Constant-Quantity ?X)
(<= (Quantity.Dimension $X Identity-Dimension) (Dimensionless-Quantity $X)) (<=> (Dimensionless-Quantity ?X) (And (Constant-Quantity ?X) (= (Quantity.Dimension ?X) Identity-Dimension))) (Nth-Domain Magnitude 3 Dimensionless-Quantity) (Forall (?Q ?Unit ?Mag) (=> (And (Constant-Quantity ?Q) (Unit-Of-Measure ?Unit) (Dimensionless-Quantity ?Mag) (Defined (* ?Mag ?Q))) (= (Magnitude (* ?Mag ?Q) ?Unit) (* ?Mag (Magnitude ?Q ?Unit))))) (=> (= (Magnitude ?Q ?Unit) ?Mag) (Dimensionless-Quantity ?Mag)) (Nth-Domain Magnitude-In-System-Of-Units 3 Dimensionless-Quantity)