noun
valley fold (origami)
A basic origami fold where the paper is folded inward, creating a valley-like crease. Contrasts with 山折り (mountain fold).
See also: 山折り
折り紙の基本は谷折りと山折りです。
The basics of origami are valley folds and mountain folds.
点線に沿って谷折りにしてください。
Please make a valley fold along the dotted line.
山折り is a mountain fold (folded outward, peak up), while 谷折り is a valley fold (folded inward, trough down). They are complementary basic folds in origami.
Compound of 谷 (tani, 'valley') and 折り (ori, 'fold'), referring to the valley-like shape of the crease.