The # symbol

I find it interesting that here are many different names for the # symbol (and symbols that look like it).

Here are some of them.

Hash

This is what I would call the symbol. It has lots of uses in computing.
I'm glad that Twitter went with "hashtag" rather than any other version.

Number

This is an American usage, although (as you might expect) it is found in other places too.
This usage is what the Unicode/HTML entity for # is named after - # - the "number sign". Wikipedia doesn't like the usage of it to mean number though, perhaps due to Wikipedia's global audience?
Number sign

Pound

This is a weird one, and apparently only rarely used outside the US. In the UK, pound (weight) is denoted by 'lb', and the monetary pound by '£'. On UK keyboards, the £ symbol is above the 3, which is where # is on a US keyboard - perhaps this is where the confusion comes from?
Pound sign

Sharp

In music, (although it's slightly different as it tilts to the left) it signifies the note to which it is applied is played a semitone higher (as opposed to flat (♭), which means played a semitone lower).
Sharp (music)

Octothorpe

I'm not really sure who uses this name.

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I possibly vaguely remember this when doing some computing back in the 80s. Not sure what computer I was using then.
Viewdata#Keypad symbols