Definition
A number that expresses how much a given material changes in length per unit of original length for each one-degree change in temperature. Each material (aluminum, steel, titanium, etc.) has its own coefficient, which engineers use to predict how much a part will grow when heated or shrink when cooled.
Plain English
A figure that tells you how much a material stretches or shrinks lengthwise when its temperature goes up or down by one degree. Different materials have different values, so a steel rod and an aluminum rod of the same length will not change size by the same amount when heated.
Context Anchor
Seen in aircraft materials and maintenance discussions when temperature can affect the length or fit of metal parts.
Derivation
Coefficient comes from Latin roots meaning 'working together' -- here, a number that works together with temperature change to give you the result. Linear simply means 'along a line' (length, not volume). Expansion is from Latin expandere, 'to spread out.' Put together: a number that, combined with a temperature change, tells you how much something spreads out lengthwise.
Why Pilots Care
Engine parts, exhaust systems, and airframe structures heat up and cool down constantly. If two joined materials expand at very different rates, the joint can loosen, warp, or crack. Designers choose materials and clearances with these coefficients in mind, which is why running an engine outside its temperature limits or skipping warm-up procedures can damage tightly-fitted parts.
Analogy
A metal ruler on a hot day becomes a tiny bit longer, even though you may not see it. The coefficient of linear expansion is the number that tells how much that length change will be for that material.
Grounding Statement
Heat a one-meter aluminum rod by 100 degrees and it grows about 2.3 millimeters; the coefficient of linear expansion is the small number that lets you calculate that growth.
Intuition Check
This does not mean the total amount a part expands. It means the rate of length change for that material for each degree of temperature change.
Example Sentence 1
The exhaust manifold is bolted with slotted holes because the steel's coefficient of linear expansion causes it to grow noticeably as it heats up in operation.
Example Sentence 2
Different metals expand at different rates, so the coefficient of linear expansion helps predict how parts will behave in changing temperatures.