hashtag
n.— «What does it mean when somebody puts a number sign before a word in an online post? That’s called a hashtag, for the hash mark preceding the word (right, yet another name for the symbol we also call the number sign and the pound sign). People on sites like Twitter use these labels to make it easier for others to spot their postings.» —“Fast Forward’s Help File” by Rob Pegoraro Washington Post Feb. 21, 2009. (source: Double-Tongued Dictionary)
Further reading
Mary Beth in Greenville, South Carolina, wonders: Why do we say four-oh-nine for the number 409 instead of four-zero-nine or four-aught-nine? What are the rules for saying either zero or oh or aught or ought to indicate that arithmetical symbol...
Astronauts returning from space say they experience what’s called the overview effect, a new understanding of the fragility of our planet and our need to reflect on what humans all share as a species. A book about the end of the universe...