Normal strain ($e_{nn}=\Delta \ell /\ell$)

The fractional change in the length of a material line in a specified direction. For infinitesimal deformation with unit direction $\mathbf{n}$, it is $\mathbf{n}\cdot \mathbf{\epsilon }\mathbf{n}$, where $\mathbf{\epsilon }$ is the small strain tensor.