Unio Trium Nationum (Latin for "Union of the Three Nations") was a pact of mutual aid codified in 1438 by three Estates of Transylvania: the (largely Hungarian) nobility, the Saxon (German) burghers, and the free military Szeklers.
Read more about Unio Trium Nationum: Medieval Administrative Structure in Transylvania, Events Leading To The Union, Brotherly Union (Union of Kápolna), Founding of The Union of Three Nations