This work is the result of contributions from 52 active researchers in geophysics, astrophysics and astronomy. We have followed a philosophy of directness and simplicity, while still allowing contributors flexibility to expand in their own areas of expertise. They are cited in the contributors’ list, but I take this opportunity to thank the contributors for their efforts and their patience. The subject areas of this dictionary at the time of this writing are among the most active of the physical sciences. Astrophysics and astronomy are enjoying a new golden era, with remarkable observations in new wave bands (γ -rays, X-rays, infrared, radio) and in new fields: neutrino and (soon) gravitational wave astronomy.