A strain is a normalized measure of deformation representing the displacement between particles in the body relative to a reference length.
Applicable units are those of quantitykind:LengthRatio
http://en.wikipedia.org/wiki/Deformation_(mechanics)
A strain is a normalized measure of deformation representing the displacement between particles in the body relative to a reference length.
$\xi = \frac{\Delta l}{l_0}$, where $\Delta l$ is the increase in length and $l_0$ is the length in a reference state to be specified.
http://www.iso.org/iso/catalogue_detail?csnumber=31889