The word retrieval (e.g., spectral retrieval, spectroscopic retrieval, atmospheric retrieval, or abundance retrieval) is now used in astrophysics for the determination of parameters (such as composition, temperature, and abundances) regarding a astronomical bodies (such as stars, brown dwarfs, and extra-solar planets) through a combination of observation, particularly spectrography, and producing or adjusting models so as to match the observations. It implies some difficulty and is commonly used for analysis of the atmospheres of exoplanets. Such retrieval consists of a backward model, often requiring determination of the parameters that allow a simpler forward model to produce observation data from the parameters. The gist of the retrieval is providing something better than random guessing of parameters.
Codes to carry out the process are termed retrieval codes.