There is a need for a reliable and comprehensive reference source providing the definitions of all the concepts, from the oldest to the newest, used in astronomy and astrophysics. Considerable efforts have so far been made to gather a large number of astronomical concepts. Despite their valuable utility, these references, however, do not meet the requirements of completeness and up-to-dateness not to mention other considerations, including the fact that they are not interdisciplinary oriented, nor freely accessible. The present Etymological Dictionary of Astronomy and Astrophysics, created at Paris Observatory, tries to meet this need. Moreover, a semantic tool, called AstroConcepts, appended to the dictionary provides definitions of astronomical concepts present on Web pages. Thanks to this tool, if one selects an astronomical concept on a web page, a pop-up window will display the definition of the available English or French terms. Another tool provides a concept map, or conceptual diagram, of each concept by displaying relationships with other concepts. A Working Group, set up at the IAU commission 46, aims at developing the dictionary into a multilingual online Dictionary of Astronomical Concepts.