I'd assume it would just be manufacture specific hardware generation, with the generation number being synchronized between manufacturers a few generations back for marketing purposes. Maybe it used to be technological leaps, but now its when a redesigned console comes out. So the switch would be the next gen after the Wii U which is the next gen after the Wii, and the Xbox Series is the next gen after the Xbox One, with literarions withing the generations being seen as decimal point increases.
That being said, Nintendo has for teh past few generations been following a tick-tock cycle where they have a banger of a console, then one they try something new or boost the power of only for it to flop, then they fix the main problems that flop had and make another banger. The Gamecube was more powerful but wasn't approachable, the Wii was just as powerful but simplified the controls, the Wii U had more power but a dizzying number of controller configurations, and the Switch combined the Wii U's controllers into a universal set.