the.com/law of one price

means An economic principle stating that in efficient markets without barriers or transport costs, identical goods should sell for the same price everywhere once converted to a common currency.

from Not an ancient coinage but a modern economic formalization — the phrase crystallized in 20th-century price theory and trade economics, naming a logic traders had long exploited in practice: if the same bushel of wheat sells for less in one market than another, savvy buyers rush in, buy low, sell high, and that very rush closes the gap. The 'law' part is economists' habit of dressing up tendencies as laws, borrowed from physics-envy common in the field — it describes what arbitrage pressure pushes toward, not an unbreakable rule.