The accusative case of a noun is the case used to mark the direct object of a verb. The same case is used in many languages for the objects of (some or all) prepositions.
The accusative case exists (or existed once) in all the Indo-European languages (including Latin, Sanskrit, Greek, German, Russian), and the Finno-Ugric languages.
English, which lacks declension in its nouns, has an explicitly marked accusative case in a few pronouns (e.g. "whom" is the accusative case of "who", and "him" is the accusative case of "he"). (Contrast with dative case, the indirect object.)
Note: who/whom and he/him are not only examples of nominative/accusative relationships in English, but also of nominative/dative. (Consider: I gave him the present, etc.) (In Old English, they were distinct - him was the dative, hine the accusative.) This duality is one of the reasons many students of English do not consider the dative to be distinct from the accusative in English -- as such, neither is an ideal term. Instead, objective is often used, to distinguish from the nominative, which is often (in the context of English grammar) called simply the subjective. English morphologically distinguishes only one case, the possessive case -- which in reality is not a case at all, but a clitic (see the entry for genitive case for more information). With a few pronominal exceptions, the objective and subjective always have the same form.
Compare nominative case, dative case, ergative case, genitive case, vocative case, ablative case.
See also: Declension