nanometer

The nanometre sits at the boundary where the everyday rules stop working. A silicon crystal's atoms are spaced about 0.543 nm apart, so a 10 nm film is fewer than twenty atoms thick. Light itself is measured here: the 589 nm sodium doublet colours street lamps orange, chlorophyll absorbs hardest near 430 nm and 662 nm, and a 405 nm laser is what a Blu-ray disc reads with. Anything smaller than a few hundred nanometres is invisible to an optical microscope no matter how good the lens, because you cannot resolve detail much finer than the wavelength you are looking with.

One nanometre figure in wide circulation is not a length at all. Semiconductor "nodes" like 5 nm or 3 nm stopped corresponding to any physical dimension on the chip years ago and now function as generation names set by the manufacturer. Real transistor features on those processes are considerably larger, so comparing node numbers between foundries compares marketing labels rather than geometry. When a nanometre figure in a datasheet matters, check what dimension it actually names.