The word "hashtag" is a neologism for the # symbol, sometimes called the pound symbol in American English (but separate from the British Pound symbol, £ ). More properly called an octothorpe, the # is anything but new; and actually dates to the Roman Empire.
The "@" symbol, now commonly called "at," was originally used to take the place of any word beginning with "a."
☛ The little pointing-hand symbol, used in some typographic lists, is properly called a manicule, and is sometimes referred to as an index or fist.
If this kind of lexico- and typographic trivia interests you, you'll enjoy the "Shady Character" blog.
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