The term "aeronomy", coined and introduced about 60 years ago, refers to the scientific study of the upper atmospheric regions of the Earth and other solar system bodies. It covers the chemistry, dynamics and energy balance of both neutral and charged particles. Thus, it requires a thorough knowledge of the basic concepts of the physics and chemistry of low density gases and plasmas. It also addresses the relevant coupling processes from above (magnetosphere) and below (mesosphere) this region of interest. Another way of thinking about aeronomy is that it is the study of the "transition region" between the lower atmosphere and space.