The name of the case used for the citation form of nouns in an ergative language. Most languages have either a nominative-accusative pattern of case marking, or an ergative-absolutive one. (In rare cases both.)

Typically the absolutive form is unmarked (has no overt case morpheme or ending.)

The absolutive is sometimes called nominative or absolute. If the former, the contrast is between nominative-accusative and nominative-ergative marking. The term absolutive is less familiar but perhaps clearer.

A noun is put in the absolutive case when it's the subject of an intransitive sentence, and when it's the direct object of a transitive sentence. See under typology for a fuller explanation of systems used across the world's languages, with examples; see also Basque for more details of a particular language using it.

By illustration, here are two Basque sentences, the first intransitive and the second transitive:

Gizona joan da 'The man goes'
Emakumeak gizona ikusi du 'The woman sees the man'
The word gizona 'the man' is subject of the first and object of the second, and is in the absolutive case: whereas the transitive subject emakumea 'the woman' is marked with the ergative ending -ak.