The onshore market of a currency is the market in its own country, under its regulators and controls; the offshore market is the market outside it, among non-residents, where the currency trades on different terms or only in cash-settled form.
Exemples
Example 18.3 (Two forward curves for one currency)
Suppose onshore USDCNY trades at 7.1000 spot and 7.0400 three months forward, and offshore USDCNH at 7.1200 and 7.0700, with a dollar rate of 3.68%. The onshore forward implies a renminbi rate of 0.31% for 91 days, the offshore one 0.89%: a basis of 0.58 percentage points, the price of moving renminbi across the line. The onshore band that day would run from 6.9580 to 7.2420.