Number sign is the preferred Unicode name for the glyph or symbol # (not to be confused with sharp ♯). The name was chosen from several used in the United States and Canada. This sign's Unicode code point is U+0023 and its ASCII value is hexadecimal 23. On a U.S. keyboard, it can be typed using Shift-3. On a Mac with a U.K. layout, Option-3 is used.
In many parts of the world, including the United Kingdom, Canada, Australia, New Zealand, Russia and the rest of Europe the name is used for the sign "№", which is Unicode U+2116 and does not appear in ASCII. It is often written simply as No. or using a superscript No. Unicode calls this sign the Numero sign to avoid confusion.
In the United States of America, the symbol is traditionally called the pound sign. It derives from a series of abbreviations for pound avoirdupois, which is a unit of mass (although pound is generally used as a unit of weight; for more information, see pound-force and pound avoirdupois). At first "lb." was used; later, printers got a special font made up of an "lb" with a line through the ascenders so that the "l" would not be mistaken for a "1". Unicode character U+2114 (℔) is called the "LB Bar Symbol," and it is a cursive development of this symbol. Finally came the reduction to two horizontal and two vertical strokes.
Its traditional commercial use in the U.S. was such that when it followed a number, it was to be read as 'pounds': 5# of sugar. And when it preceded a number, it was to be read as 'number': #2 pencil, which still appears on U.S. pencils. Thus the same character in a printer's type case had two uses.
1 Other Names
2 In other languages
3 See also
4 References (as numbered above)
It has many other names (and uses) in English. (Those in bold are listed as alternative names in the Unicode documentation.)
from its use in many shell scripts and some programming languages like Perl to introduce comment text
Ken Moody, lecturer at the University of Cambridge, used to call it "chickenscratch"
possibly used by computer programmers, as parallel to ! (exclamation mark), which programmers often call "bang". These two characters are used together in shell scripts. The parallelism stems from cosmological physics, which posits that the universe began in a "big bang" and may end in a "big crunch".
fence, gate, grid, gridlet
hash / hash mark / hash sign
the most common name outside the U.S., including in the UK, Australia, and New Zealand.
this is the name used by UK computer professionals.
Used in the UK, Australia, and New Zealand on touch-tone telephones – "Please press the hash key"
In the UK the symbol is often used as medical shorthand for 'fracture' 
from its use to denote hexadecimal values in some markup and programming languages
introduced in the tonsil of the Intercal reference manual, and often reproduced in The Hacker's Dictionary
William Sherk in 500 Years of New Words (1983), p. 272, has the following entry: "Octothorn, The number sign (#); so called because there are eight points, or thorns, sticking out of it ..."
octalthorpe / octothorp / octothorpe / octatherp
See wiktionary:Octothorpe for etymology. With some detail at www.octothorp.us.
See http://doug.kerr.home.att.net/pumpkin/index.htm#Octatherp for detailed alternate etymology.
The Merriam-Webster New Book of Word Histories, 1991, has a long article that's consistent with Doug Kerr's essay, in that it says octotherp was the original spelling, and that the word arose in the 1960s among telephone engineers.
http://www.worldwidewords.org/weirdwords/ww-oct1.htm tells of three possible etymologies, none likely, and says it wasn't in print until 1974, so the Merriam-Webster story that says it appeared in the 1960s may be more credible.
pound / pound sign
Used as the symbol for the pound avoirdupois in the U.S. (where lb. would be used in the UK and Canada; note that lb. or lbs. is common in the U.S. as well and is used by the general public more often than #). It is never called a "pound sign" in the UK, where that term always denotes the symbol for pounds sterling (£) rather than that for pounds weight (lb).
Keith Gordon Irwin in, The Romance of Writing, p. 125 says: "The Italian libbra (from the old Latin word libra, 'balance') represented a weight almost exactly equal to the avoirdupois pound of England. The Italian abbreviation of lb with a line drawn across the letters was used for both weights. The business clerks' hurried way of writing the abbreviation appears to have been responsible for the # sign used for pound."
Used in the U.S. and Canada on touch-tone telephones – "Please press the pound key"
resemblance to the glyph used in music notation; so called in the name of the Microsoft-invented programming language, C#. However Microsoft says at Frequently Asked Questions About C#:
It's not the "hash" (or pound) symbol as most people believe. It's actually supposed to be the musical sharp symbol. However, because the sharp symbol is not present on the standard keyboard, it's easier to type the hash ("#") symbol. The name of the language is, of course, pronounced "see sharp".
Since most fonts don't contain the sharp sign most websites will doubtless continue to use the fallback hash mark. The music sharp sign, which should be used if available, is U+266F (♯).
In computing a shebang is the inexact contraction of sharp and bang the typical names of the # and ! signs used at the beginning of executable text files.
used by editors to indicate where space should be inserted in a proof. This can mean (1) a line space (the space between two adjacent lines denoted by line # in the margin), (2) a hair space (the space between two letters in a word, denoted by hr #) (3) a word space, or letter space (the space between two words on a line, two letter spaces being ##). Em- and en-spaces (being the length of a letter m and n, respectively) are indicated by a square-shaped em- or en-quad character ( and ̷, respectively).
colloquial term referring to vague resemblance of # to a squashed spider
sometimes used for the asterisk (*), for the same reason
sometimes used for the cloverleaf-like Command key on keyboards for Apple Macintosh computer keyboards
occasionally used in the UK (e.g. sometimes in BT publications and automatic messages) - especially during the Prestel era, when the symbol was a page address delimiter
the International Telecommunications Union specification ITU-T E.161 3.2.2 states: "The # is to be known as a 'square' or the most commonly used equivalent term in other languages."
tic-tac-toe (U.S.) / noughts-and-crosses (UK)
resemblance to game board
In a URL the sign is used immediately after the URL of a webpage or other resource to introduce a "fragment identifier" — a name or id which defines a position within that resource or a section of the document. For example, in the URL http://en.wikipedia.org/wiki/Number_sign#In_other_languages the portion after the # (In_other_languages)is the fragment identifier. A relative reference to the fragment from within the document itself can start with the number sign, and consist of just the fragment identifier: <a href="#top">TOC</a> refers to an anchor named "top" on the current web page. 
In other languages
Arabic: "مربع", pronounced "morabaa" (square)
Bulgarian: диез, pronounced dies
Chinese: "井號" (jǐng hào; literally: "well" sign) as it resembles the hanzi for water well (井; jǐng)
Czech: mřížka (small grid or small bar)
Danish: firkant (square), the official name used by telcos for touch-tone key, or havelåge (garden gate from the gate in a picket fence)
Dutch: hekje (picket fence)
Estonian: trellid (grate)
Finnish: ruutu (square) or risuaita (brushwood fence)
French: dièse (sharp sign)
German: Raute or Rautenzeichen (Rhombus, the official name used by telcos for the touch-tone key), Lattenzaun (picket fence), Doppelkreuz (double cross)
Greek: δίεση (diesis)
Hebrew: סוּלָמִית, pronounced sulamit (from sulam == "ladder" + -it, feminine ending)
Hungarian: kettőskereszt (double cross)
Italian: cancelletto (small gate)
Japanese: "番号記号" (bangōkigō, "number sign"); "井桁" (igeta, literally the rim of a well, which is traditionally this shape) or "シャープ" (sharp in katakana)
Norwegian: firkant (square) or (sometimes) skigard (A particular kind of fence with a visual resemblance)
Persian: "چهارگوش" (Four-Corner) pronounced "Chaa'haa'r Goo'sh"
Polish: hash, krzyżyk (sharp sign), the former used by IT professionals and the latter by telecom
Portuguese: cardinal, used in mathematics notation to represent the cardinality of a set, sustenido ("sharp sign"), jogo-da-velha ("tic-tac-toe"). Also known as "cerquilha" in Brazil. because it resembles a picket fence (cerca).
Romanian: diez ("sharp sign")
Russian: решётка (reshëtka), pronounced rye-SHOT-ka (grid)
Spanish: numeral, cuadradillo, almohadilla ("cushion"), sostenido ("sharp sign"), michi, gato, tatetí (the last three meaning "tic-tac-toe")
Slovak: mriežka ("small grid")
Swedish: stakettecken (fence sign), "staket" (fence), "galler" ("grid, grating") fyrkant (square), ruta ('square, box') or brädgård (timberyard)
Swiss German: Gartehag (fence)
Thai: เครื่องหมายสี่เหลี่ยม (square sign, only used in telephone service)
Turkish: Sayı işareti