To answer this question coherently and sensibly, we need to clarify the terminology.
The defining characteristic of the Zen school of Buddhism is the use of meditation to achieve satori. For this reason, Zen temples pay particular attention to gardens, which serve to aid concentration and meditation.
The idea of gardens as a tool for relaxation and therefore meditation in general is attributed to Musō Soseki (1275–1351).
Zen aesthetics are dominated by seven characteristics that I have described elsewhere.
The most important is the absence of symmetry. The second is the pursuit of naturalness. The so-called Zen garden is the Karesansui, a garden where stones symbolize islands and raked gravel symbolizes water. The position of the stones is not random; on the contrary, it requires great care. For example, the garden of the Ryōanji temple in Kyoto is constructed in such a way that none of its stones are visible from any position. At least one is invisible. This is apparently to demonstrate how reality cannot be understood through the senses. However, it should be borne in mind that this type of garden is only one of several types of Zen garden and that Zen gardens can also be found in temples of other schools.
This is a garden in the Kenchōji Zen temple. It is undoubtedly a Zen garden, because it was built according to the Zen aesthetic standards I mentioned, but it is not a karesansui.

And this one?

This is undoubtedly a karesansui. The temple to which it belongs is not Zen, but Jōdo. It doesn't matter, it is Zen nonetheless, precisely because it was built according to Zen aesthetic standards.
The Zen school has always been sponsored by the state and is even part of the state. As a result, its intellectual creations have been hugely successful and its aesthetics have become the Japanese aesthetic par excellence.
The situation can therefore be viewed in two ways.
On the one hand, the Zen label applied to architecture has lost all meaning. Its characteristics can be found practically everywhere. This type of bell-shaped window, for example, is widely used by everyone and is of Zen origin.

On the other hand, however, it is undeniable that the Zen school has redefined Japanese culture and the way it expresses itself. We must remember the role it has played in Japanese history.
Conclusion: a Zen garden is a garden built according to Zen aesthetic standards. To find out what these are, follow the link above.
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