Meaning (VCT 1969, Article 2(1)(a)): a treaty is "an international agreement concluded between States in written form and governed by international law, whether embodied in a single instrument or in two or more related instruments and whatever its particular designation" — the last phrase the essential lesson: treaty, convention, covenant, charter, pact, accord, modus vivendi, exchange of notes, agreed minutes — names vary, the legal nature is one: an agreement governed by international law intended to create legal rights and obligations.
Types:
1. By parties: bilateral (the 1950 Nepal–India Treaty) vs. multilateral (the UN Charter, UNCLOS);
2. By function: contract-treaties (an exchange: the trade treaty) vs. law-making treaties (general norms: the Geneva Conventions; the human-rights covenants);
3. By openness: closed (restricted membership), open (accession by all states), framework conventions (protocols to follow);
4. By duration and form: definitive vs. provisional (modus vivendi); single-instrument vs. simplified-form agreements (exchange of notes, oral agreements — outside the VCT's written definition but not outside international law).
Importance: treaties are the modern system's primary source (PIL-I, Unit 2), the constitution- makers of international life (the UN Charter), the state's daily instrument (Nepal's existence as a treaty-participant: borders, trade, transit, rivers, labour), and the legal face of sovereignty's exercise — consent made visible.