means A formal, written agreement between two or more sovereign states or parties, setting out terms each side commits to honor.

from From Latin tractatus, "a handling, management, discussion," from tractare, "to drag about, handle, deal with" — the same root that gives us "tract" and "treatment." It reached English in the 14th century through Old French traité, carrying the sense of a matter formally talked over and settled. So a treaty is, at heart, a thing that has been thoroughly handled — gripped, hauled into shape, and dealt with at the table.