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Thermal expansion coefficient is a material property that describes how much the size of a substance changes with a change in temperature. It is typically expressed as a ratio of the fractional change in size to the change in temperature. For most materials, the volume, area, and length all increase as temperature increases. This phenomenon is known as thermal expansion. Conversely, when temperature decreases, most materials contract.

The linear thermal expansion coefficient, often denoted by the Greek letter alpha (α), specifically refers to the change in length of a material per unit length per degree Celsius (or Kelvin) change in temperature. The volumetric thermal expansion coefficient, denoted by beta (β), describes the change in volume per unit volume per degree Celsius. For isotropic materials, the volumetric coefficient is approximately three times the linear coefficient (β ≈ 3α).

The value of the thermal expansion coefficient varies significantly between different materials. Metals generally have higher thermal expansion coefficients than ceramics or polymers. Understanding and accounting for thermal expansion is crucial in many engineering applications, such as the design of bridges, railways, and pipelines, to prevent stress and deformation caused by temperature fluctuations. Conversely, some applications, like precision instruments, require materials with very low thermal expansion coefficients to maintain dimensional stability.