ghost
n.— «The two are known as “ghosts,” in police parlance, or undercover officers whose job is to keep an eye on and provide back-up to a third undercover officer who is doing the buy-and-bust work.» —“Man Shot Dead by Police on Lafayette” The Local (Fort Greene, Brooklyn, New York) July 11, 2009. (source: Double-Tongued Dictionary)
Further reading
Imagine telling someone how to get to your home, but without using the name of your street, or any other street within ten miles. Could you do it? We take street names for granted, but these words are useful for far more, like applying for a job or...
If you speak a second or third language, you may remember the first time you dreamed in that new tongue. But does this milestone mean you’re actually fluent? And a couple’s dispute over the word regret: Say you wish you’d been able...