“5 nm node is expected to have a contacted gate pitch of 51 nanometers and a tightest metal pitch of 30 nanometers” - wikipedia

Its a marketing term

A nanometer is a well defined physical dimension. If we split a milimeter in a million parts, each one is a nanometer

Sure, but (very unfortunately for clear communication) that's not what "5 nm" written by brancz in "Apple silicon is at 5 nm" means. "Apple silicon is at TSMC N5" would be more accurate.

Yes, it’s a real unit, but when used by silicon manufacturers it’s a marketing term that doesn’t correspond to any physical dimension.