Organisms that can interbreed and produce fertile offspring are called a species.

Here's why:

* Interbreeding: This means they can mate and produce offspring.

* Fertile offspring: The offspring must also be able to reproduce themselves. This is key because if the offspring are sterile, it indicates a lack of genetic compatibility and a separate species designation.

Example: Horses and donkeys can interbreed and produce mules. However, mules are sterile, meaning they cannot have offspring. Therefore, horses and donkeys are considered separate species.

Let me know if you'd like more examples or have other questions!