The term 'mark (# symbol)' is a reference to the HASH symbol, which is a four-letter character used in computing and telecommunications. It is also known as the number sign, pound sign, or octothorpe. The HASH symbol is used to indicate the beginning of a comment line in many programming languages, and it is also used to separate parts of a URL. In addition, it is often used to denote a hashtag on social media platforms.