1 Simple molecules [molecule: A molecule is a collection of two or more atoms held together by chemical bonds. 2  It is the smallest part of a substance that displays the properties of the substance.
Eon has a number of meanings. In Astronomy, an eon refers to 1 billion (109) years. But it also refers to a very long, unspecified period of time, or specific geologic stages of the Earth.
As defined by Dictionary.com, an eon can either refer to an indefinitely long time period such as an historical age, or specifically one billion years when used as a measurement of time in astronomy. When used geologically, it refers to the former definition, comprising two or more eras.