Engagement
Viewport
Also called visible window.
Viewport is the visible rectangle of the page in the browser or device. The fold, scroll depth and viewability all rely on it.
How it is measured
Read the window's inner width and height. Tools group the results into device classes. Zoom and browser chrome change the number without changing the screen.
The viewport is not screen size. A resized desktop browser and a phone in landscape can have the same viewport.
Worked example
A local museum finds 58 percent of visits arrive at 390 px width. Its ticket panel breaks below 400 px, so the buy button wraps off screen.
A layout fix lifts mobile ticket clicks by a third.
How it differs
The viewport is the window. Above the fold is what it shows first. Scroll depth measures how far it moves down the page. It differs from screen size, which describes the hardware.
Common errors
Using screen size instead. Ignoring zoom. Testing in a resized desktop window. Forgetting orientation. Assuming common sizes.
In practice
Open the top three viewport sizes from your own data and test each of your key pages in them.