Term: Ergative Verbs

The sun melted the butter.

Here, we have a sentence with a standard subject, a transitive verb and a direct object.

The butter melted.

Here, the subject was the object of the original sentence. The butter didn't melt itself- it required the heat of the sun. This is an ergative verb use, where the subject of the intransitive form of the verb would be the object of the transitive form of the verb.