The # symbol is commonly called the pound sign, number sign and more recently the hashtag. It is called the pound sign because the symbol comes from the abbreviation for weight, lb, or “libra pondo” literally “pound by weight” in Latin. When writing lb, it was not uncommon for scribes to cross the letters across the top with a line across the top, like a t. See the example below.
The phrase “number sign” arose in Britain because “pound sign” could easily be confused with the British currency. The # symbol is sometimes spoken as the word “number” as in the word “number two pencil.”
But what is its official name? The octothorpe. What does that mean? It’s actually a made-up word. It was invented in the same laboratories where the telephone came from. The scientists at Bell Laboratories modified the telephone keypad in the early 1960s and added the # symbol to send instructions to the telephone operating system. Since the # symbol didn’t have a name, the technicians made one up. They knew it should be called “octo-” something because it has eight ends around the edge. But how to make “octo” into a noun? What happened next is not entirely clear. According to one report, Bell Lab employee Don MacPherson named it after the Olympian Jim Thorpe. Another former employee claims it was a nonsense word that is a joke. Another unverifiable report is much more etymologically satisfying. The Old Norse word “thorpe” meant “farm or field”, so octothorpe literally means “eight fields.”
The word hash predates these other terms but was not very popular until recently. (Maybe because it reminds us of mediocre diner food.) It first referred to stripes on military jackets as early as 1910. In the 1980s, it came to refer to the # symbol. Since the ascent of social media, hashtag has become the favored word for the # symbol.
Similar symbols appear in many other places. Musicians recognize # as the sharp symbol, denoting a note one half step higher. Copy editors see a symbol meaning “space,” as in “add a space between two sentences.” In computer code, the # symbol means that everything that follows is only comment, not instructions.
*http://blog.dictionary.com/octothorpe/
