Reign: Noun

Definition: the period of time during which someone or something is the best or the most important, powerful, etc.

source: http://www.merriam-webster.com/dictionary/reign

in the article ” The happiest and Saddest Places in NYC, According to Twitter” by Brian Merchant it states ” Then they created a map that reveals where angry tweets reign…”

Now that I know the meaning of this word I know that they created a map that was able to tell them where the greatest angry people where.