The Collaborative International Dictionary of English v.0.48:

octothorp \oc"to*thorp\, octothorpe \oc"to*thorpe\, n. [octo-
   eight + thorp Etymology of thorp uncertain. (ca. 1965). See
   quote below. Possibly derived from octalthorpe or octotherp
   (once used by the Bell System?).]
   A typographic symbol (#) having two vertical lines
   intersected by two horizontal lines. It is also called the
   crosshatch, hash, numeral sign and number sign; in
   the U. S. it is commonly called the pound sign, especially
   to designate the symbol as used on digital telephone dials,
   but this can be confusing to Europeans who think of the pound
   sign as the symbol for the British pound. It is commonly used
   as a symbol for the word number; as in #36 (meaning: number
   thirty-six).
   [PJC]

         octothorp
         Otherwise known as the numeral sign. It has also been
         used as a symbol for the pound avoirdupois, but this
         usage is now archaic. In cartography, it is also a
         symbol for village: eight fields around a central
         square, and this is the source of its name. Octothorp
         means eight fields.
                                                  --Robert
                                                  Bringhurst
                                                  (The Elements
                                                  of Typographic
                                                  Style (2d
                                                  edition,
                                                  1996), Hartley
                                                  & Marks,
                                                  Publishers,
                                                  Point Roberts,
                                                  WA; Vancouver,
                                                  BC, Canada, p.
                                                  282)
   [Joel Neely]