What is the Coefficient of Thermal Expansion? The Coefficient of Thermal Expansion, also known as CTE, is the degree to which a given material expands when it is heated. Depending on what material you are working with, that material will have a specific CTE that differs from other materials. When heat is applied to a substance, the distance between the individual atoms that make up that substance increases. This leads to an overall expansion of the material dependent upon the number of atoms involved, and therefore its size. Knowing the standardized CTE of a material allows you to account for any expected expansion when conducting measurements or using the material to build parts.