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Accretion rate is a general term for measures of accretion (the enlargement of an object by the addition of material), typically mass per unit time. The term may be used for any astrophysical instance of accretion, e.g., stars, planet formation and black holes (black hole accretion rate).
An astronomical object's Eddington accretion rate (Eddington limit) is a theoretical upper bound on accretion rates, consisting of that at which the object reaches its Eddington luminosity. The limit is relevant to active galactic nuclei (AGNs) and quasars which are presumed to be the result of supermassive black hole (SMBH) accretion (AGN accretion), the limit providing hints to the size of the black hole. It appears that SMBH accretion was most widespread within the history of the universe around the time of maximal star formation, the peak star-formation epoch. A conundrum in astrophysics is evidence suggesting SMBH accretion exceeded the Eddington limit during the early universe.
The terms accretion and accretion rate are also used in meteorology and may be found in planetary science. One example of the use of the terms is the growth of water droplets (or other liquid droplets) in the formation of clouds and rain. Another is the addition of water to a wetland through precipitation.