From 'hash' (#) + 'tag' (label) — coined 2007 by Chris Messina for Twitter, initially rejected, then globally adopted.
A word or phrase preceded by the symbol # that classifies or categorizes the accompanying text on social media.
A compound of 'hash' (the # symbol, called 'hash mark' in British English) + 'tag' (a label). The use of # for categorizing content on social media was proposed by Chris Messina on Twitter in August 2007. 'Hash' in this sense may come from 'hatch' (cross-hatching — the # symbol resembles cross-hatched lines). 'Tag' from Middle English 'tagge' (a dangling piece, a label), of uncertain origin. A hashtag is literally 'a hash-mark label' — a #label. Key roots
The # symbol has different names in different contexts: 'hash' (British), 'number sign' (American), 'pound sign' (US telephony), 'sharp' (music — though the actual sharp sign ♯ is different), and 'octothorpe' (a whimsical Bell Labs coinage of uncertain origin). Chris Messina proposed using it for Twitter categorization in 2007. Twitter initially resisted the idea — then adopted it, and '#' became one of the defining symbols of the internet age.