Octothorpe

What do you call the # symbol on your phone? The pound sign? The number sign? The hash sign? The octothorpe?  
 
The pound sign #, meaning a weight in pounds, comes from Latin libra pondo, abbreviated as lp, which ended up in English as ‘lb’. If you look carefully at ‘lb’, you can almost see a #. The term ‘pound sign’ is from US usage in the 1930s.
 
The number sign #, meaning a number, has its origins in an 1853 treatise on bookkeeping. In an 1890s typewriter manual, # was known as the ‘number mark’. Later sources refer to the ‘number sign’. [Bonus factoid: Bookkeeping is the only word in English with three consecutive double letters].
 
What is the difference between # as a pound sign and # as a number sign? When # is used after a number, it means a weight in pounds; e.g., a 5# bag of flour = a five-pound bag. On the other hand, when # is used before a number (e.g., a #2 pencil), you would say, “A number 2 pencil.”
 
What about the # symbol in music referring to the term ‘sharp’; e.g., C# means ‘C sharp’. The # is used because of its resemblance to the glyph ♯ used in musical notation.
 
Hashtag? Hash, meaning a stew of meat cut into small pieces, is from the 1660s. Hash, meaning a mess, is from 1735. Hash marks on a football field were first called as such in 1954.
 
The hash sign #, in computing, is from 1979.The use of hashtags in social media was first proposed in 2007. A ‘hashtag’ is the message that follows the hash sign. In 2014, the Oxford English Dictionary defined hashtag as “a word or phrase with the symbol # in front of it, used in social media websites and apps so that you can search for all messages with the same subject.”
 
How about calling the # symbol an octothorpe? This term was first used by the tele-communications industry in 1971 in reference to the # symbol on a phone following the advent of touch-tone dialing in the 1960s. Octo- refers to the eight points on the # symbol. The origins of the -thorpe part of octothorpe are unknown. Thorpe is an Old English word for a village or hamlet, if this helps at all to explain the term!
 
When I was a kid, # simply meant the start of a game of x’s and o’s or ‘tic tac toe’.
 
Image: The abbreviation written by Isaac Newton, showing the evolution from "℔" toward "#".
 
Reference: Online Etymological Dictionary, https://www.etymonline.com/
https://en.wikipedia.org/wiki/Number_sign
https://en.wikipedia.org/wiki/Hashtag