Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature. When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. This results in an increase in the overall dimensions of the material.

The coefficient of thermal expansion (α) describes how the size of an object changes with a change in temperature. Different materials have different coefficients, with metals generally having higher values than ceramics or glass. Understanding thermal expansion is crucial in engineering applications where temperature changes occur.

Thermal expansion must be considered in many engineering applications. Bridges often have expansion joints to accommodate length changes. Railroad tracks have small gaps between sections to prevent buckling in hot weather. Bimetallic strips, which bend due to differential expansion, are used in thermostats and circuit breakers.

In precision instruments and aerospace applications, materials with low thermal expansion coefficients (like Invar) are used to maintain dimensional stability. Conversely, thermal expansion is exploited in shrink fitting, where a metal ring is heated to expand, fitted over a shaft, and then cooled to create a tight mechanical joint.