Market Letter
  
Think of it like a newsletter...but focused purely on markets. It's a way to distribute advice about some segment of the market. The Jones Gold Daily...or The Wise Investor's Guide to Indonesian Debt.
Market letters might be distributed as a subscription product (i.e., an investor pays $25 a month to receive a daily list of five Nigerian princes looking for help). Or it could be sent out as a kind of loss leader: a marketing tactic for a broker or other market-adjacent firm (i.e., "sign up for a brokerage account at Tony Bradford & Sons and you'll receive our weekly market letter detailing the best places to invest your money"...HINT: the best place to invest your money is always in one or another of Tony Bradford's proprietary mutual funds).