I have not come across a 'definition' as such, but would not any system or equipment whose failure can cause significant environmental damage qualify as ECE?
If a gas compressor trips and that results in venting hydrocarbons to the atmosphere, or an oil catcher releases oil into a river or the sea, a benzene pump, chlorine or ammonia compressor seal leaks, they qould also qualify as ECE, I guess.
But why are you after this info? After all, any failure that poses a risk needs maintenance action. Some risks relate to Safety, Health or the Environment, others to Production Loss or Asset damage. If these risks are not tolerable, we need suitable action.