The symbol # is called the number sign, hash, octothorp and sometimes the pound sign (not to be confused with the currency symbol of the same name). Historically, it has had use for several purposes such as showing an ordinal number and the pound as a unit of mass.
Since 2007, the symbol became famous after it was used in hashtags across social media.
This is the name chosen by the Unicode Consortium, which is most common in Canada and the United States.
In Canada and the United States, the # key on a phone is called the pound sign, pound key, or simply pound. This name is rarely used elsewhere because it can refer to the currency symbol £.
In the United Kingdom and Australia, it is often called a hash because it resembles cross-hatching.
The term appeared in South African writings from the 1960s.