A measure of the relative change in the size of a body. Linear strain is the change per unit length of a linear dimension. True (or natural) strain is the natural logarithm of the ratio of the length at the moment of observation to the original gauge length. Shearing strain is the change in angle (expressed in radians) between two reference lines originally at right angles. When the term is used alone, it usually refers to linear strain in the direction of the applied stress.