The symbol # is most commonly known as the number sign, hash, or (in North American usage) pound sign. The symbol has historically been used for a wide range of purposes, including the designation of an ordinal number and as a ligatured abbreviation for pounds avoirdupois.

Since 2007, widespread usage of the symbol to introduce metadata tags on social media platforms has led to such tags being known as "hashtags", and from that, the symbol itself is sometimes called a hashtag.

The symbol is distinguished from similar symbols by its combination of level horizontal strokes and right-tilting vertical strokes.

The number sign is often used in information technology to highlight a special meaning. It was adopted for use within internet relay chat (IRC) networks circa 1988 to label groups and topics. This usage inspired Chris Messina to propose a similar system to be used on Twitter to tag topics of interest on the microblogging network. Although the hashtag started out most popularly on Twitter as the main social media platform for this use, the use has extended to other social media sites.

The word hashtag is often used when reading social media messages aloud, indicating the start of a hashtag. For instance, the text "#foo" is often read out loud as "hashtag, foo" (as opposed to "hash, foo"). This leads to the common belief that the symbol itself is called hashtag. Twitter documentation refers to it as "the hashtag symbol".

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