At the moment this glossary section contains only very few keywords. It will be further extended within the next few months.
Astrometry is the branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. More information is found on Wikipedia.
ESA (European Space Agency) is an intergovernmental organisation dedicated to the exploration of space, currently with 20 member states. For information is found on Wikipedia and the ESA web page. The official Gaia web page at ESA is found here.
Hipparcos (High Precision Parallax Collecting Satellite) was a scientific mission of the European Space Agency (ESA), launched in 1989 and operated until 1993. For more information see the Wikipedia article and the ESA Webpage.
Interferometer: An array of telescopes or mirror segments acting together in order to reach a high angular resolution by the superposition of (electromagnetic) waves. More information on astronomical interferometers is found on Wikipedia.
Photometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation (e.g. light). Besides the photometry performed with the main astrometric instrument Gaia contains a photometer, which in the strict sense is a low-resolution spectrograph for two different wavelength regions (BP = Blue Photometer = Blaues Photometer, RP = Red Photometer = Rotes Photometer). More information on astronomical photometry is found on Wikipedia.
Spectroscopy uses the dissection of electromagnetic radiation (in particular of optical light) into its wavelengths. From the investigation of stellar spectra many properties of stars can be derived, such as their chemical composition, temperature, their gravitational acceleration at the surface, and their (radial) motion along the line of sight. Gaia contains two spectroscopic instruments: The photometer, which in the strict sense is a low-resolution spectrograph for two different optical wavelength regions (BP = Blue Photometer = Blaues Photometer, RP = Red Photometer = Rotes Photemeter) and the near infrared spectrograph RVS (= Radial Velocity Spectrograph) with higher resolution whose main task is to measure the radial (along the line-of-sight) velocities of stars. For more information on astronomical spectroscopy see the Wikipedia article.