I believe 'hash' comes from Twitter's use of # for their 'hashtag' function (basically keywords to track/search tweets about a particular topic).
The term 'pound' was at least in use for telephones when switching over from rotary dials to touch tone buttons, and the # and * (star) buttons were added... Wikipedia says it is often claimed the number sign
evolved from the pound symbol (℔ written in cursive handwriting, cleaned up and simplified). Maybe it's not a coincidence they are on the same key for UK keyboards.
