Meaning for ECO light in Nissan cars dash board?

Not 100% sure, but my Honda also has an ECO light on the dash. My vehicle is equipped with Variable Cylinder Management (VCM) that will automatically shut down 1-3 of the cylinders during cruising speeds thus, improving fuel economy. When this happens the ECO light comes on to let me know that I am driving with fewer cylinders activated. See if your vehicle is equipped with VCM or check your owners manual.