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Strain measures the fractional deformation of a body — the ratio of change in dimension to original dimension. Being a ratio of like quantities, strain is dimensionless (no units, dimensional formula [$M^0$ $L^0$ $T^0$]).

Three types correspond to three types of deformation. Longitudinal strain (epsilon = Delta L/L) measures extension or compression along the length. Shear strain (gamma = theta, the angle of shear in radians) measures angular deformation — valid as tan(theta) approximately theta for small deformations. Volumetric strain ($epsilon_V$ = Delta V/V) measures fractional volume change.

Important distinctions: (1) Longitudinal strain can be positive (tensile) or negative (compressive). (2) Shear strain changes shape but not volume. (3) Volumetric strain changes volume but not shape. (4) When a wire is stretched, all three types of strain occur simultaneously — it gets longer (longitudinal), thinner (lateral), and its volume changes slightly. The relationships between these are governed by Poisson's ratio.