Inverse length The inverse of length - frequently used for absorption or attenuation coefficients and wave numbers. This can also be used to for extrinsic curvature where the unit 'diopter' is used. Space and Time QUDT http://qudt.org/vocab/quantity#Inverselength This should not be used for linear densities, but only for quantities with a truly dimensionless numerator. For linear densities, use "Count per length" ODM2 Units Type Controlled Vocabulary ODM2 Working Group A vocabulary for describing the type of the Unit or the more general quantity that the Unit represents.