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1.
The pound sign will appear as # in Latin script and indicates that what follows is should be read as a number in some series.

May also be referred to as a hash especially in computing circles or outside of North America or number sign.

Referring to a pound sign as a hashtag will probably, rightly or wrongly (but probably rightly) give cause for adults to think you are a child.
To access your bank account, please press the pound sign, followed by your PIN number.
by Stormin'Norman June 14, 2013
 
2.
A tattoo on the lower back of a female. Generally some kind of tribal tatt, seen on the wall of a tattoo shop, or a butterfly, dolphin, or sun burst. Usually seen just above the thong line, to draw attention to it.
Damn bro, did you see that pound sign on that chick at the club last night?!
by Rooster2188 January 08, 2010