Whale is the industry term for a small number if individuals that spend large sums of money on individual titles, commonly used to reference targets of Free-to-play or microtransaction based schemes.
It's because a small percent of any given population spends a disproportionately large amount of money on those microtransactions.
If making a standard product is akin to fishing, then making a product with microtransactions is akin to whaling, where you're get fewer overall catches but each individual catch is infinitely more profitable.