A raven is one of several larger-bodied species of the genus Corvus. These species do not form a single taxonomic group within the genus.
There is no consistent distinction between "crows" and "ravens", and these appellations have been assigned to different species chiefly on the basis of their size, crows generally being smaller than ravens.
The largest raven species are the common raven and the thick-billed raven.
The term "raven" originally referred to the common raven, the type species of the genus Corvus, which has a larger distribution than any other species of Corvus, ranging over much of the Northern Hemisphere.
The modern English word raven has cognates in all other Germanic languages, including Old Norse (and subsequently modern Icelandic) hrafn and Old High German (h)raban, all of which descend from Proto-Germanic *hrabanaz.
Obsolete collective nouns for a group of ravens (or at least the common raven) include "unkindness" and "conspiracy". In practice, most people use the more generic "flock".