I prefer to call the symbol “#” a crosshatch, as it is in many dictionaries. But this symbol has gotten quite a beating over the years since the computer age some decades ago.
There is now incredible confusion over what to call it. When interacting with Ma Bell, the telephone company calls it a “pound sign”. How on Earth could “#” be confused with “£”? I understand that “£” appears in place of “#” on a British computer keyboard (both are SHIFT+3). In its place, “#” is on American keyboards, in use even in Canada. My keyboard is a US keyboard, and “£” is a special character that I have to conjure up with some kind of HTML hocus-pocus. “££££££” (OK, so now I’m cutting and pasting). I would think, however, that the British have “#” on their telephone keypads; and the prospect of a British “#” being mistaken for a “£” would likely make them cringe. There is an indication that it is likely to literally mean pounds of weight in the old Imperial/US system, so that 12# could stand for “12 pounds”, for instance.
Now we have Twitter, and now everyone is calling it a “hashtag”. Well, yes, it is a hash (a kind of crosshatch) that tags a message to another one so a sender can stay under the character limit. It is certainly a hashtag if you use Twitter-speak, but that is because it is a symbol that has meaning to the computers and humans which run amok in the Twitter-verse.
To help things, some phone messages are now asking you to press “the number sign”, which is less wrong I suppose, and I am happier to press a “number sign” than I would be to press a “pound sign”, unless it is actually a “£” sign. If you actually call it a crosshatch, and say in your phone message “Press crosshatch to continue”, few people would know what you are talking about. This is just sad, and reveals the awful truth that most people really don’t know what to call it. Conversely, we all know that it is a symbol which represents a number or a quantity, so the simplest thing to say to a stranger is “number sign” when you mean “crosshatch”.
And some people who want to be really fancy call “#” an “octothorpe”. According to the Oxford English Dictionary, this word was invented in 1973 by Bell Labs precisely to describe the “#” character on their newfangled touch-tone phones. The word octothorpe did not appear in the OED until the Fourth Edition (2004), and says that the origin is uncertain, but defaults to the claim by Bell Labs. Another website seemed to say that an octothorpe used to refer to 8 parcels of land surrounding a central square section of land. So that would make the property lines kind of like this: #. The most common spelling of octothorpe is without an “e”, but the OED says it’s OK to use it with one. By the way this backstory has never been confirmed, only claimed.
But you hardly hear of “octothorpe”, since there are so many other terms for it. The OED calls it a “hash sign”; and I’ve already mentioned “crosshatch”, “pound sign”, and “number sign”. There can also be “hash symbol”, and Twitter calls it a “hash tag”, but according to the OED, my preferred term is not exactly used for “#” (although other dictionaries say so). Instead the OED, spelling it “cross-hatch” (with a hyphen), says it can be used to mean two pairs of parallel lines crossing each other (kind of like #, but referring to the pattern generally).