is a product sold at a low price (at cost or below cost) to stimulate other profitable sales. It is a kind of sales promotion, in other words marketing concentrating on a pricing strategy. A loss leader is often a popular article. Sometimes leader is now used as a related term and means any popular article, in other words one sold at a normal price.
The extreme couponer is someone who purchases things only when they are on sale as a loss leader and then they bring the price of that item down with coupons. The extreme couponer rarely ever pays full price or falls victim to the bait and switch.