The word "约束" is a compound formed by combining two characters, "约" and "束", which are synonymous in this context, both carrying meanings related to binding and restriction. "约" (yuē) originally meant "to tie" or "to make an agreement", thus implying a bond or limitation, while "束" (shù) directly means "to bind" or "to tie up". When used together, they reinforce each other to create a single, more emphatic concept meaning "to restrain", "to constrain", "to restrict", or "a binding restriction", effectively describing the act or state of being held in check by rules, agreements, or physical limitations.