https://drafts.csswg.org/cssom-view/#layout-viewport has:

The visual viewport is a kind of viewport whose scrolling area is another viewport, called the layout viewport.

However, while this is the definition of the "layout viewport" term, it doesn't actually define what it means at all.

It should instead use "actual viewport", as defined in https://drafts.csswg.org/css-viewport-1/#actual-viewport. (This definition is also very minimal, but that's tracked by #206.)

These terms might be changed, see https://drafts.csswg.org/css-viewport-1/#issue-44630653 ("ISSUE 3: Make actual viewport the layout viewport, define visual viewport.") and #4819.