"Octothorpe" is one of many names for the # key - usually found below the 9 and to the right of 0 on a touch-tone phone. It's also called the tick-tack-toe sign, cross-hash, cross-hatch, enter, hash, number-sign, noughts-and-crosses, octothorp, pound, pound-sign and probably other things.
Over here, British Telecom insists on calling it the "square" key.