What is a variation or strain created using horticultural or agricultural practices that are not generally seen in the natural population referred to as?

A cultivar is a plant species variation or strain created through horticultural or agricultural practices such as selective breeding or genetic engineering. 

Cultivars differ from naturally occurring variations within a species in that they have been selected for specific characteristics such as flower colour, fruit size, or disease resistance. 

The term "cultivar" is a contraction of "cultivated variety," and it is used to distinguish these man-made plants from their wild relatives. Cultivars are an important part of modern agriculture because they allow farmers to grow plants with specific desired traits, increasing crop yields and quality.