I'll just go ahead and do this for all of those of us who aren't linguists.

Main Entry: er·ga·tive
Pronunciation: '&r-g&-tiv
Function: adjective
Etymology: Greek ergatEs worker, from ergon work
Date: 1939
: of, relating to, or being a language (as Inuit or Georgian) in which the objects of transitive verbs and subjects of intransitive verbs are typically marked by the same linguistic forms; also : being an inflectional morpheme that typically marks the subject of a transitive verb in an ergative language