The number sign (#) is called an an octothorpe, octothorp, octathorp, or even octatherp.
(In the United States, the symbol is usually called the pound sign, and the key bearing this symbol on touch-tone phones is called the pound key. But in the UK it is never called the pound sign since the term "pound sign" is understood to mean the currency symbol, £, and it is never used to refer to pounds as a unit of weight either, instead lb is used. The symbol is most often called the hash. Basically it's a confusing little sign.)