We call each of the observables a dimension and the unit of measurement the unit of that dimension. The water in a glass on a table has the dimensions of volume, mass, temperature, density, pressure, surface area, velocity, etc. Others, such as the mass and temperature, are not related since they can be independently varied. In the former case, the unit of density must equal the ratio of the units of mass and volume, while in the latter no relationship exists between the units of mass and temperature. Furthermore, physical laws place additional constraints on the independence of measurements.
Physical laws equate observables of one kind to others.
Each term in the equation representing a physical law must have the same dimension.