热胀冷缩是指物体受热时会膨胀，遇冷时会收缩的特性热胀冷缩是由于物体内的粒子（原子）运动会随温度改变，当温度上升时，粒子的振动幅度加大，令物体膨胀；但当温度下降时，粒子的振动幅度便会减少，使物体收缩。

热胀冷缩是一般物体的特性，但水（4°C以下）、锑、铋、镓和青铜等物质，在某些温度范围内受热时收缩，遇冷时会膨胀，恰与一般物体特性相反。因此，水结冰时，冰是先在水面出现。由于铁轨有热胀冷缩的特性，因此铁轨链接时须保持一定的间隙（以防止气温升高时，铁轨因受热膨胀伸长而相互推挤变形），再以鱼尾钣与螺杆将铁轨相互链接起来。

Thermal expansion is the tendency of matter to change in shape, area, and volume in response to a change in temperature, through heat transfer.

Temperature is a monotonic function of the average molecular kinetic energy of a substance. When a substance is heated, the kinetic energy of its molecules increases. Thus, the molecules begin moving more and usually maintain a greater average separation. Materials which contract with increasing temperature are unusual; this effect is limited in size, and only occurs within limited temperature ranges (see examples below). The degree of expansion divided by the change in temperature is called the material's coefficient of thermal expansion and generally varies with temperature.