In the field of physics, linear density or density per unit length — generally represented by the letters of the Greek alphabet: λ (lambda) or μ (mu) — refers to the measurement of a physical magnitude or property per unit length.[1] Two typical examples used in science and engineering are linear mass density (amount of mass per unit length) and linear charge density (amount of electric charge per unit length).[2]​.

The phrase linear density is most often used when describing the characteristics of one-dimensional objects, although linear density can also be used to describe the density of a three-dimensional quantity along a particular dimension. Just as density is most often used to refer to mass density, the term linear density also often refers to linear mass density. However, this is just an example of a linear density, since any quantity can be measured in terms of its value along a dimension.[3]​.

In relation to fibers and yarns, the mass unit of length is defined by Denier "Denier (unit)") or tex (mass unit of yarn length used in the textile industry).[4]​.