/ˈhæʃtæɡ/
hash·tag
BrE /ˈhæʃtæɡ/
NAmE /ˈhæʃtæɡ/
a word or phrase with the symbol ’#’ in front of it, included in some messages sent using the Twitter social networking service so that you can search for all messages with the same subject
I often use hashtags to search for trending topics.