Mechain was a medieval cantref in the Kingdom of Powys. This cantref has also been referred to as Y Fyrnwy (Vyrnwy). Mechain may owe its name to the River Cain which flows through it on its way to join the River Vyrnwy; 'Me' or 'Mach' (c.f. Machynlleth, Mathrafal, etc., and in modern Welsh, field is maes) may signify meadows or plain, in which case Mechain would mean "Meadows of the Cain". It corresponds to the later hundred of Llanfyllin.