One can define his/her target market by looking at the following;

Looking at your current market customer base.

Studying one's customer base helps in identify the common characteristics and interests of the customers and the reason why they consume your product. It is possible that other people with the same interests could benefit from the product.

Looking at your competitors

You can look at their current market and their target market. You can decide to find another market that the competitors overlook which becomes your target market.