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"hashtag" is the word that goes with the "#"; the symbol itself is just the hash character (number sign):
http://en.wikipedia.org/wiki/Hashtag
http://en.wikipedia.org/wiki/Number_sign
Worse thing is that in French we commonly use the sharp/dièse word for the hash symbol (as here "#" is not used for "number", we use "n°" instead), so they officially translated "hashtag" to "mot-dièse"...
http://en.wikipedia.org/wiki/Hashtag
http://en.wikipedia.org/wiki/Number_sign
What about the sharp/dièse/diesis sign in music? In somewhat different writing for purists, but most certainly the origin of all these signs... 
Worse thing is that in French we commonly use the sharp/dièse word for the hash symbol (as here "#" is not used for "number", we use "n°" instead), so they officially translated "hashtag" to "mot-dièse"...