hashtag
The octothorpe is used in the US to signal a unit within a block at a particular address.
The octothorpe has been popularized on Twitter for use in hashtags.
Some people believe that the # sign depicts eight homesteads around a village common, hence the name octothorp.

SMART Vocabulary: related words and phrases

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