Like Agos says, thermal decomposition definitely starts happening below 160°C
I spent a lot of time playing around trying to make Caramel a less dangerous process (ever try making 20lbs of caramel when you're doing it at 160°C? Good luck :D)
In 2017, I came up with an alternative caramel process that never goes above 125°C
It takes hours instead of minutes, ...
Often, cooking onions over low heat so they turn translucent without browning would be sweating as was already mentioned, but the name would technically mean cooking them until they gave up their liquid.
With a little bit of water in the pan, and the lid on, I'd be inclined to call it steaming, but if you then cooked it without the lid some to get rid of the ...
Without the water, I'd call it 'sweating'.
With the water & lid, they're going to come out a bit like like fairground hotdog onions… not really like caramelised.
Fine for Spanish Tortilla (or hot dogs), not so great if you need that Maillard-like reaction.
If you don't use the water, or the lid, you can slowly caramelise onions with little supervision, ...