In the past there has been some ambiguity—primarily in the spoken word—attached to the meaning of such terms as “thermal strain”. There is thus a need to standardise terminology. It is proposed that the following definitions be adopted generally. These are common to standard text books and have been adopted as a National Aerospace Standard in America and by the Royal Aircraft Establishment.

The definitions are not phrased for the mathematician, but for the engineer who deals with normal engineering materials. Materials which are not homogeneous or isotropic, for example, require more detailed mathematical definition.