The hash sign. In the USA it is sometimes called 'the number sign', but this is confusing for speakers of BE (who use "no" - the more confusing abbreviation of the Latin numero). It may be more confusing, but it's what we're used to; and it would be less confusing if I could make the little o superscript. In the absence of a superscript font, some people use a full-stop (period) to distinguish it from the word 'no'.
Even more confusing for speakers of BE is another AmE expression 'the pound sign'.