Hash sign # (pound sign, number sign)

• Number sign: “#1 seller,” “Apt. #4B.” Read it as “number.”
• Pound sign: the name on US phone keypads and in phone menus (“press pound”). It is usually traced to the old abbreviation for pounds of weight. In the UK, “pound sign” means £ instead.
• Hash and hashtag: on social media, #topic groups posts by subject.
• Octothorpe: a name coined at Bell Labs in the 1960s for the telephone keypad key.

Position changes the meaning in older US usage: before a number, # means “number” (#5 is number five); after a number, it means pounds (5# of flour is five pounds). You still see the second form in old recipes and shipping notes, but “lb” is clearer.

• ♯ (music sharp) is a separate character. In ♯ the vertical strokes are upright and the horizontal strokes slant upward; in # the horizontal strokes are level. Use ♯ in music notation.
• C#, the programming language, is officially written with the number sign, even though it is pronounced “C sharp.”
• № (numero sign) is the “number” abbreviation used in Russian and some European text.
• ＃ (fullwidth number sign) belongs to Chinese, Japanese and Korean fullwidth text.

If you are writing for an international audience, “No. 5” is clearer than “#5,” because outside North America many readers know # only as hash or hashtag.

• Comments: # like this in Python, Ruby, shell scripts and YAML.
• Markdown: # at the start of a line makes a heading; ## makes a smaller one.
• CSS: #header selects the element with that id, and #ff6600 is a hex color.
• URLs: everything after # is a fragment that jumps to part of a page. Browsers don’t send it to the server.
• C and C++: preprocessor directives like #include.
• Excel: errors such as #N/A, and a cell full of ##### when the column is too narrow for its number.