hashtag
A number sign (#) displays only significant digits.
Authentication fails if the username begins with the number sign.
I was in such a rush to send the letter, I used number signs where I should have put apostrophes.

SMART Vocabulary: related words and phrases

Signs, signals and symbols
acid test
backward slash
backwards slash
beacon
breadcrumb
hieroglyphic
hieroglyphically
iconographic
iconographically
iconography
obelus
octothorpe
paraph
penalty flag
pilcrow
pointer
prompt
the all-clear
token
torch