This online dictionary presents the definition of classical as well as advanced concepts of modern astronomy. Moreover, each English entry is accompanied by its French and Persian equivalents. The dictionary is intended to be helpful to professional as well as amateur astronomers. As a notable particularity, it also provides a detailed etymology of English and Persian terms. The etymological material contained in this work may interest linguists, in particular those concerned with the evolution of Indo-European languages, especially with that of their Iranian branch. Apart from educational and outreach objectives in the field of astronomy, one of the main aims of this work is to contribute to the Persian language by creating a comprehensive dictionary of astronomy and astrophysics.