Zairai is a Japanese tea term that I heard occasionally when I first got into tea but it took a few years for me to learn what it means. As I'm sure you know all tea comes from the same plant. Simply put, zairai is a variety of tea that isn't a recognizable cultivar. You may also see it referred to as a yamacha. The word is translated as "native" and is generally used to describe older tea trees that are grown from seed.
This allows for a lot more genetic variation so there are differences in leaf color, shape, harvest times, and flavor from plant to plant. These trees are not exactly wild like those you might find in Yunnan province but they are feral, meaning that humans have not interfered with their growth for some time. Genetically speaking they are more closely related to the plants that were imported from China in the 14th century.
Modern tea farming practices generally use cloning to cultivate specific varieties in order to ensure that the plants in a particular section will have similar traits. Cultivars are often selected for their resistance to frost, pests, and other environmental threats. In Japan, there are somewhere around 200 registered cultivars of tea. Zairai is unpopular with farmers because of the unpredictability it brings. The budding time will be different even for plants that are right next to each other, making it more difficult to harvest by machine.
The yield is also significantly lower than Yabukita and other cultivated varieties. Yabukita accounts for 75% of all of the tea produced today. Zairai makes up just 2%. For that reason, it can be quite hard to find even within Japan. Although I've written about several hundred Japanese teas over the years I have never had the opportunity to try one. That is something I need to change soon!