Power-saving mode. I know Acer used this in their notebooks. (I had one.) You could manually switch to a reduced power mode. Because it used less battery power it was advertised as eco-friendly since I guess the marketing assumed conservation always means the same thing.

Nowadays every CPU and peripheral does automatic power throttling all the time and no one pretends it has anything to do with being environmentally conscious.

I presume it indicates an "eco-friendly" power mode.

Eco mode?