One of many ways of referring to the symbol '#'. In my experience, this is the most common term refering to this symbol, although it has the disadvantage of also refering to £, the sign for the Britsh pound (the money, not the unit of weight).
# means 'number' ("#3") or, in editing, 'insert an extra space here'.
Other names for this symbol include:
Octothorpe
Delimeter
Cross-hatch
Gridlet
Double-cross
Tic-Tac-toe
And, perhaps most obscure of all, the Cross of Lorainne