Hashtag, pound sign, hash or octothorpe?
The # character has many names. Its official Unicode name is number sign, from uses like #1 or "apartment #4". Americans also call it the pound sign, because it was used as an abbreviation for pounds of weight and probably developed from the ℔ symbol. In the UK and much of the world it is a hash, and engineers at Bell Labs nicknamed it the octothorpe in the 1960s for the new touch-tone phone keypad.
Strictly speaking, a hashtag is the # plus the word that follows it (#travel). Chris Messina suggested grouping posts this way on Twitter in 2007, and the idea spread to Instagram, TikTok and almost every other platform.
How to write good hashtags
A hashtag ends at the first space or punctuation mark, so write #ThrowbackThursday, not #Throwback Thursday. Letters, numbers and underscores are safe, while symbols like & or - cut the tag short. Capitalize each word (CamelCase): it is easier to read and helps screen readers pronounce the words correctly. A tag made only of digits, such as #2026, usually does not become a link.
Only the plain # reliably turns into a link. The music sharp ♯ looks similar, with slanted crossbars and upright vertical strokes, but apps treat it as an ordinary symbol. Musicians should still use ♯ for sharps (C♯); see music symbols.
How to type # and where else it is used
On a US keyboard, # is Shift + 3. On a UK Mac keyboard, Shift + 3 gives £, so use Option + 3; UK PC keyboards have a separate # key next to Enter. With Alt codes, hold Alt and type 35. On phones, # is on the 123 or ?123 keyboard, and the #️⃣ emoji is in the Symbols section of the emoji keyboard.
In programming, # starts a comment in Python, Ruby and shell scripts, selects an id in CSS (#menu) and begins a hex color (#ff0000). In a URL it points to a section of the page, and in Markdown a line that starts with # is a heading. For mentions, see the at symbol; for № and other number signs, see number symbols.