In linguistics: a transformation of sentences in an ergative language. It is similar to, or parallels, the passive voice of more familiar accusative languages.

The passive transformation in European languages applies to a sentence that has two actants, a subject and an object. It promotes the object to subject and demotes the old subject to a minor role, where it can be omitted:

The person saw the dog.
--> The dog was seen (by the person).

The antipassive, like the passive, changes a transitive construction into an intransitive one. Where the basic sentence had both subject and object, the transformed one has only a subject, together with an optional element in some oblique (non-object) form.

In an ergative language like Basque or Inuktitut (Eskimo), the simplest marking is the absolutive, which is often zero, and this is also used for the object of a transitive sentence. Here are examples from Inuktitut:

Inuk takuv-uq. 'The person saw.'
Inu-up qimmiq takuv-aa. 'The person saw the dog.'

In the first the subject inuk has no ending, nor has the object qimmiq 'dog' in the second. The first is intransitive (has only one actant) so the verb takes the ending -uq. The second is transitive so the verb ends in -aa. This is the usual way of saying it in Inuktitut.

The antipassive is like a passive in that it promotes the marked actant (in this case the transitive subject) into the unmarked case, and demotes the old unmarked form into an oblique case.

Inuk qimmir-mik takuv-uq. 'The person saw a dog.'

It also now has the intransitive verb ending -uq. There is only one actant, inuk 'person', that has any effect on the verb.

Note also the subtle shift in meaning. 'The person saw the dog' has two definite actants. There are no actual articles 'the' or 'a' in Inuktitut, but the demotion of qimmiq 'dog' is also a demotion in definiteness. In a similar way the English passive demotes the importance of the identity of the old subject: The dog was seen (by someone, who need not be mentioned).

Some linguists use the term 'antipassive' in a looser way for other reductions in transitivity, e.g. she studies as compared to she studies biology.