Cumulative Layout Shift (CLS)
How much the page moves under the reader while it loads. Good is 0.1 or less; the usual cause is content that arrives without a reserved place.
An image without width and height, a font that swaps to a different size, an advertisement that appears above the text — each of them shoves what the reader was looking at. The measure is deliberately about the reader, not the machine: a shift after a click does not count, because the reader caused it.
The fix is always the same: reserve the space before the thing arrives. Sizes on media, a height on every slot that may fill later, and fonts that fall back to a face of similar metrics.