According to Wikipedia [1], it refers to "the process' minimum feature size."

[1] https://en.wikipedia.org/wiki/Semiconductor_device_fabricati...

In many cases, like in Samsung case, it does not even refer to the feature size, and just represents a generation of the tech process or some arbitrary incremental improvements.

Literally nothing more than a marketing "group name" for a family of processes, none of which have any dimension approaching 3nm.

This talks a little about how the meaning has changed over time:

https://en.wikichip.org/wiki/technology_node

Probably the size the lightwave smallest etch on a die. Now an etch doesn't make a transistor which means the smallest useful etch is larger.