A class of animals or plants that have the same characteristics and are able to breed together?

That's a great description of a species.

Here's a breakdown:

* Shared Characteristics: Members of a species share many physical and behavioral traits.

* Interbreeding: They can reproduce with each other and produce fertile offspring.

This ability to interbreed is key to defining a species. If two groups can't produce offspring, or if their offspring are infertile, they are considered separate species.