The official name of the number sign, the one located at the bottom right hand corner of phone keypads and used to label hashtags on social media, is octothorpe or octothorp. It is also called a hash or a pound sign. Since 2007, the symbol has enjoyed widespread usage to introduce metadata tags on social media platforms which led to such tags being known as "hashtags".

The octothorpe's definition, according to the Merriam-Webster dictionary, claims the word's "octo" prefix refers to the eight points on the popular symbol, but the "thorpe" remains a mystery. One theory claims that it comes from the Old English word for "village", based on the idea that the symbol looks like a village surrounded by eight fields.

Many scholars believe the word was invented by workers at the Bell Telephone Laboratories by 1968. The symbol, along with the asterisk, was first added to telephones in order to make the keypad a neat square. Those two symbols were chosen since they were already common on standard typewriter keyboards, but at that time, the # still didn't have a technical name. It was first called an "octotherp", which eventually became the octothorpe.

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