The standard unit of currency in the Agatean Empire. In its usual form as a pure gold coin, it can be divided into half-rhinu and quarter-rhinu pieces, for ease of making small purchases.
In Ankh-Morpork, when the currency was exported there by Twoflower, "small purchases" available to a one-rhinu coin might have included two or three districts of the city with an option on the Patrician's Palace. This caused a certain amount of civic unrest which was indirectly responsible for one of the city's Great Fires. It also led to Rincewind getting a lesson in reflected-sound-of-underground-spirits (echo-gnomics) from the then Patrician, which he totally failed to grasp.
While Ankh-Morpork is largely over the inadvertent destruction caused by Twoflower and a travelling chest full of solid gold coins, another innovation from the Counterweight Continent, first encountered by Rincewind while buying dubious foodstuffs from Disembowel-Meself-Honourably Dibhala is poised to arrive in the city. This is higher-order rhinu currency, which to Rincewind's consternation and disbelief comes in the form of pieces of paper which claim to be worth the gold weight of five and ten rhinus respectively. Rincewind believes it will never catch on. But then, we've seen that his grasp of economic theory is every bit as abysmal as his grasp of magical theory. Ankh-Morpork's cash economy and currency are, however, in the hands of people who do understand economics, and changes are poised to happen...