means The art of clipping shrubs and trees into ornamental, often fanciful shapes.

from From Latin topiarius, 'an ornamental gardener,' which itself grew out of Greek topia, 'a place' (the root of 'topic' and 'topography') — by way of topion, 'a little place,' a diminutive used for decorative landscape scenes. So buried in the word is the idea of garden-as-painted-place: the Romans had topiarii who sculpted greenery into miniature landscapes, and English borrowed the term, fully formed, around the late 16th century.