The term used to refer to the breeding of improved varieties of plants and animals is selective breeding.

Here's why:

* Selective breeding is a process where humans intentionally choose organisms with desirable traits to reproduce, aiming to enhance those traits in future generations.

* This process is also known as artificial selection, emphasizing the human intervention in the breeding process, unlike natural selection where nature determines which organisms survive and reproduce.

Therefore, selective breeding is the most accurate term for describing the intentional breeding of improved varieties of plants and animals.