federative

Concise Oxford English Dictionary © 2008 Oxford University Press:
federate

▶verb /ˈfɛdəreɪt/ (of a number of states or organizations) organize or be organized on a federal basis. ▶adjective /ˈfɛd(ə)rət/ relating to such an arrangement.

– derivatives
federative adjective.

– origin C18 (as adj.): from late L. foederatus, based on foedus, foeder- (see federal).