Suzerainty is a term which was originally used for the relation between the feudal lord and his vassal; the lord was said to be the suzerain of the vassal, and at that time suzerainty was a term of Constitutional Law only. With the disappearance of the feudal system, suzerainty of this kind likewise disappeared. Modern suzerainty involves only a few rights of the suzerain State over the vassal State which can be called constitutional rights. The rights of the suzerain State over the vassal are principally international rights, of whatever they may consist.
Source: wiktionary