The word itself has 3 possible definitions as it relates to Argentine Tango:
1.) A Musical Style or Genre which is typically 2 beats in a 4 beat measure time signature.
2.) A particular Style of Dance which matches the style of music. Historically speaking the MUSIC of Milonga (which predates Tango music) created the dance and style of music that we think of today. However, it should be noted that Milonga ‘Portena’ is last iteration of the Style of Milonga music and could not have created Tango as we know it today. No, that honor goes to its progenitor Milonga Criolla (pron: cree-oh-ja). Further it should be noted that Milonga Criolla begat the idea that would become Milonga ‘Porteña’ which comprises several Milonga ideas including Milonga Lisa, and then later on Milonga Traspie.
3.) The Event that we dance at, a Tango dance party.