Although the word treaty can be etymologically traced back to the Latin tractus, meaning treatment, handling, discussion, and management, there was no Latin word with that root having the notion of an (international) agreement. If anything, tractus sometimes had the sense of a disagreement, of a violent handling of affairs, such as the dragging by the hair of the priestess of Apollo. A common Latin word for treaty is foedus—interestingly a word that also meant the unseemly, horrible, and detestable, probably depicting in the mind of the users the forced circumstances and unholy power deals that led to the conclusion of some. Another Latin word for treaty is conventio, from which the English word convention derives, a term currently used as a synonym for treaty, especially when following long multilateral negotiations. Conventio has in addition the meaning of an assembly and is a word that literally translates sumbasis, an ancient Greek word for treaty. Sumbainō had the meaning of coming to an agreement but also of walking together, just like in the Latin convenio. Walking together along the same path or in the same direction is a good metaphor for agreement, though in practice it was also meant literally, like the walking together of comrades to the assembly, battlefield, or exile. There were other words for treaty in ancient Greek, such as sumphōnia, the harmony of speaking with a "common voice" about an issue, depicting people in great solidarity and symphony, or sunthēkē, which meant literally the composition of words, emphasizing the textual or synthetic sense of an agreement. But the most formal and solemn treaty was called spondē, meaning literally "libation," which included the calling of the gods to witness the treaty and the taking of the oaths that sanctified it. Unlike other agreements, the breaking of a spondē was not just an illegal or immoral act, but a sacrilege.
The move to the term treaty signifies a change from the usual metaphors of agreement but also a turn toward secularization in international relations and law. In the sense of a contract between states the first recorded use of treete is in 1430. But as a technical term of international law the word is commonly employed from the end of the seventeenth century onward (Oxford English Dictionary). Interestingly, the introduction of the term follows the end of the Thirty Years' War and the conclusion of the Treaty of Westphalia (1648), which is supposed to have secularized international norms and practice and provided the foundation of the modern (European) interstate system.
Typically, the only state that made and still makes the point of not using the name treaty for its legally binding international agreements is the Holy See. In diplomatic practice, bilateral treaties signed with the Vatican are called concordats, and canonists have gone to great pains to show that the "nature of concordats" bears practical similarities—but is not identical—to that of treaties. A concordat refers to a cordial agreement, a union of wills, the successful meeting of hearts and minds in Christian harmony. The concordat is supposed to pass its provisions spiritually, requires no diplomatic "handling," and its conclusion is avoided by the Holy See if it foresees complications in the ratification process from the other side. It is a euphemism through which papal treaty practice is rendered sui generis, supposed always to operate in concord, thus rhetorically separating itself from the worldly bargaining and crude pursuit of national interest associated with conventional treaty-making. Note also that in concordat practice, the Holy See emulates the discourse of the new covenant, which in Christian cosmology constitutes "the treaty of treaties," rendering all other sacred or secular agreements false or insignificant by comparison. It was the messianic advent of Jesus Christ that brought forth the "new covenant," bypassing and superceding the collective oath of the faithful to keep the "old covenant." As taken up in the Epistle to the Hebrews—which is precisely a call to reject the old and accept the new covenant—the old covenant requires command ethics and rabbinic enforcement, whereas the new one inscribes the divine laws in the minds and hearts of the people in concordat style (see sidebar).
The story of rendering the old covenant obsolete following the declaration of a momentous happening or new revelation has ironically established a pattern in secular treaty law, and specifically in the employment of the principle of clausula rebus sic stantibus, the "clause of things standing thus." This clause renders a treaty obsolete if there is a significant change in the conditions under which it was first concluded. The principle is a late-sixteenth-century invention coined by Alberico Gentili (1552–1608), a Protestant theologian and international jurist, in De iure belli libri tres. By mixing religious and legal discourse, oath-taking and treaty (foedus) ratification, Gentili suggests that there is in every treaty a silent assumption, an understanding (intelligitur) or mental reservation (subintelligi) of a clausula rebus sic stantibus (pp. 244–245, 599). This is only the case among Christian rulers, for Gentili doubts the legal durability of treaties concluded with untrustworthy infidel rulers (nec fidere infidelibus potes) on scriptural qua moral grounds: "For although the impious oath of an infidel may be accepted, yet what trust can be put in an unbeliever" (p. 660). That a tacit understanding of termination exists when circumstances fundamentally change reinforces the privilege of "mortal gods," but highly complicates the binding status of treaties between them. Not surprisingly, in actual diplomatic practice, statesmen from Otto von Bismarck (1815–1898) to Woodrow Wilson (1856–1924) considered the denouncement of or abrogation from a treaty as the solemn and inalienable right of state sovereignty. Taken to its logical conclusion, clausula rebus sic standibus poses a fundamental challenge to the principle of pacta sunt servanta, the treaty's defining characteristic. Still, the rebus sic standibus principle became part of customary international law and was codified—albeit in a more restrictive form because of its common abuse—in the Vienna Convention on the Law of Treaties (Article 62).