A group of people who match specific parameters that make them more likely to purchase a particular product.
The specific parameters can be...
demographic based. (age, gender, race).
location based.
values based. (they like, prioritize, care about, or dislike certain things for personal or emotional reasons)
situational based. In short, their situation in life finds them pursuing a specific goal or living in a specific situation.(eg. interested in starting a business, a college student, new home buyer, etc.)
Or a combination of the above four.
You, being the entrepreneurial type, can see the value in turning lemons into lemonade and selling it on a hot summer's day to people in your neighborhood*. The problem is, you don't have lemons or a place to sell them!