Compound
Definition
A pure substance made of two or more different elements chemically bonded in a fixed ratio. Water and table salt are compounds.
Technical definition
A compound is a pure substance formed when atoms of two or more elements bond chemically in a fixed, definite ratio, producing a substance with properties distinct from its constituent elements. It can be decomposed only by chemical reactions.
Example
Sodium chloride always contains sodium and chlorine in a 1:1 ratio. Change that ratio and the substance is no longer sodium chloride.
Common confusion
Compounds differ from mixtures: in a compound the elements are chemically bonded in fixed proportions, while a mixture's components keep their identities and can vary in ratio.