The # symbol used in social-media hashtags is commonly called a hash. It is also known as the number sign, pound sign, or hash mark, depending on region and context.
The character has a long history before social media. It appeared in early teleprinter codes and was assigned code 35 in ASCII, later becoming Unicode character U+0023. In the United Kingdom and Australia, “hash” is the usual everyday term, while “pound sign” is common in North America, where it can also refer to the currency symbol £ in informal speech.
A hashtag is technically not just the symbol: it is the hash character followed by a word or phrase, such as #science. The symbol marks the beginning of a searchable topic label. This distinction causes a common mix-up, because people often call the character itself a “hashtag.” In social-media usage, however, “hash” is a recognized and historically established name for #.