x_{1} [possibly dimensionful number] is the local density/concentration of/in x_{2} (object) per unit x_{3} at locus x_{4}, of density type x_{5}; x_{2} measures x_{1} units of x_{5} per unit x_{3} at locus x_{4}.

x_{5} might typically refer to count - not mass. Not necessarily (although possible) volumetric mass density; could be temporal, per unit area, per unit length, or other (x_{3}); could be non-mass-based (such as for electric charge or probability densities; x_{5}); relative concentration would be a count density (x_{5}) per count of another substance (x_{3}). Can be applied to number/count; thus, the units may vary and in fact the quantity might even be unitless. In a sense, intensities and fluxes are spacial-temporal densities. It is recommended that x_{2} be a physical entity/object/field or probability distribution in some sense (not merely have physical quality/meaning/value); thus, while this word could be applied to many things described with the word "per", it would probably not make much sense to call velocity a temporal displacement density (although in some contexts it might). Notice that "local" implies derivatives (Calculus differentiation).

- nildemparbi
- x
_{1}(unitless/dimensionless number; li) is the specific gravity (density or concentration ratio) of x_{2}(substance) with respect to/by standard/per unit of density/concentration of x_{3}(substance), where density/concentration of each substance is being taken per unit x_{4 }= nildenmi_{3}at locus x_{5 }= nildenmi_{4}of density/concentration type x_{6 }= nildenmi_{5}. - flaukse
- x
_{1}is the vector flux (flow) of quantity/substance (count) x_{2}through (geometric/imaginary) hypersurface/embedded manifold x_{3}per unit hyperarea per unit time. - flaumji
- x
_{1}is the scalar flux (total flow) of quantity/substance x_{2}through bounding Jordan orientable manifold x_{3}under conditions and conventions of definition x_{4}(including actual orientation of x_{3}, signum of outflux, etc.)