What does the ECO light mean in the dash of a 1993 Mitsubishi diamante?

That particular model of Diamante has two transmission modes: Economy and Power. You can switch it back and forth by pressing the button labeled "A/T Mode" near the gearshift lever. Economy mode shifts sooner, which doesn't let the engine rev as high and thus saves gas; it also loses a noticeable amount of power, especially when climbing hills. Power mode keeps the transmission in the lower gears for longer, allowing more accelerating/climbing power but using more gas.