https://drafts.csswg.org/css-grid-2/#grid-order

Grid-modified document order (grid order) is the order in which grid items are encountered when traversing the grid’s grid cells. If two items are encountered at the same time, they are taken in order-modified document order.

The definition should clarify that references to this term are supposed to specify the major axis, explain what the major axis is, and probably have a diagram similar to this one from https://en.wikipedia.org/wiki/Row-_and_column-major_order by CMG Lee (CC BY-SA 4.0, no changes made)