The word "约定" is formed by combining two meaningful characters into a single conceptual unit. The first character, 约 (yuē), originally picturing bound silk threads, carries the core ideas of "to arrange", "to restrain", or "to make an agreement". The second character, 定 (dìng), depicting stability under a roof, means "to settle", "to fix", or "to decide". When placed together, their individual meanings fuse conceptually: the act of arranging or agreeing (约) is given permanence and fixity (定), resulting in the combined sense of a "mutually settled agreement" or an "appointment", where the promise itself becomes a fixed point.