Definition
Substances made by chemically combining two or more elements in fixed proportions, producing a new material whose properties differ from those of the elements that formed it. The atoms in a compound are bonded together and cannot be separated by physical means -- only by chemical reaction.
Plain English
A compound is a substance made when two or more elements join chemically to form something new. Once joined, the parts cannot be pulled apart by simply mixing or filtering -- it takes a chemical change to break them up.
Context Anchor
Seen in aviation maintenance and aircraft systems discussions involving fuel, batteries, corrosion, cleaners, and materials.
Derivation
From Latin componere, meaning 'to put together.' A compound is literally something 'put together' from parts -- in chemistry, from elements that have bonded into a new substance.
Why Pilots Care
Pilots and mechanics may need to recognize that a compound can behave differently from the materials that formed it, especially in fuel contamination, corrosion, fumes, and aircraft-approved cleaning products.
Intuition Check
Do not read compounds here as just “several things placed together.” In this technical sense, compounds are substances whose parts are chemically joined into a new material.
Example Sentence 1
Water and carbon dioxide are compounds produced when aviation fuel burns in the engine.
Example Sentence 2
Only FAA-approved corrosion preventive compounds may be applied to the landing gear struts during the annual inspection.