There is no literal translation of wabikusa in English.
Wabi denotes something aesthetic, imperfectly beautiful and natural.
Kusa means grass, or sometimes, plant.
It denotes something natural, beautiful, sweetly imperfect and involving plants.
The English language is clearly not conceptual enough to have a single word to express all those connotations, which is probably why we have kept the name wabikusa; to maintain the simple elegance of the concepts the Japanese name denotes.
Wabikusa is a ball of substrate covered with plants that are grown in their terrestrial form, very similar to plants that are grown hydroponically in nurseries. This substrate ball is placed directly into a small glass container with some water in and then allowed to grow naturally.
This is a very different approach to aquascaping techniques, where the layout of the aquarium is planned with exquisite care and the plants are carefully maintained. Wabikusa allows nature to flow freely, and as such it produces stunning result.