Some people called it the hash symbol, and then they used it to mark tags (as in categories or keywords). So technically, the symbol is not the hashtag but the entire #tag construct is.

No idea, but it used to cause confusion that on a British keyboard Shift-3 produced the £ symbol, but a US keyboard produced the # symbol - while in the respective countries they were both referred to as "pound"

Perhaps some British influence somewhere? Where were # tags first popularised in the public imagination? Twitter I assume? Or something else?

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Edit to add side-observation:

"Octothorpe" I quite like, but I've never heard it spoken and mostly only read it in quite nerdy internetty contexts. Even amongst nerds I don't think we really use it that much.

Meanwhile @ is occasionally referred to as "commercial at" and I've seen one or two uses of "ampersat" for it. Again, I don't think I've ever heard anyone use those in speech even though they might actualy be useful for disambiguating the symbol from the word (but I think people say "at sign" when they want to be specific).

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Another passing thought:

I remember being exceedingly irritated to get a Mac with a UK keyboard in the early 2000s and discovering that the # symbol was not shifted to another key (as on a PC keyboard) but had to be accessed via the Alt-gr key combo (possibly with shift thrown into the mix as well?) ... as a recovering C++ developer at that time it was super annoying.

Nowadays I have a Swedish keyboard and while # is fine, I have similar pain with {} and [] ... and now I think of it, what is the proper name for what I exclusively call "curly braces" ?

> what is the proper name for what I exclusively call "curly braces" ?

Sideways moustaches.

> I have similar pain with {} and []

in grade school I believe we were told those were "braces" and "brackets" respectively

Perhaps influence from programmers, who would know the real "pound sign" is either ℔ or £, depending what type of pound you prefer, and the number sign is №.

£ is a stylised L with a bar, incidentally.

Yeah, it’s an interesting one. I’ve always known it as the hash symbol from back in the 80s when I got my first microcomputer. This was in the UK but I don’t think hash is UK-specific terminology: “shebang” (“hash bang” for #!) has been common parlance amongst the UNIX-crowd since I don’t know when.

I know pound is incredibly common in the US, but in the UK it’s never referred to that way because here pound means £.

“Octothorpe” is entirely new to me though, and I guess is quite context specific?

> “shebang” (“hash bang” for #!)

I think the etymology of "shebang" isn't clear, some sources say it may come from "sharp bang" or "shell bang".

http://catb.org/jargon/html/S/shebang.html

Yeah, I don't think I even heard "pound" for # until I went online and started getting exposed a lot more to US stuff (also harking from the 80s UK micro world).

I feel like "octothorpe" might have been referenced in the jargon file or similar texts?

I checked, and it gets a passing mention (along with a snipe about annoying Britons!) in Jargon File and Hackers Dictionary.