I think the actual term I used was ‘running’ or ‘movement’ economy. Economy and efficiency are related in sports science but not the same. The exact definition of economy is the energy it takes to move at any given speed. Economy incorporates both a technical efficiency component as well as a metabolic efficiency component.
Think of the technique involved in swimming for instance. A good swimmer is moves more efficiently through the water and so used less energy per meter than a poor swimmer. For metabolic efficiency you can think about fat being the principle fuel or carbs being the main fuel powering the movement. Fat burning is more ‘efficient’.
So, when we think of the economy of an alpinist we think of that person being able to move over moderate to difficult alpine terrain with less energy expenditure then some one with lower technical skills. This requires lots of technique work just as the swimmer had to to do to hone his technique. And we can think of this alpinist as being highly fat adapted so that he can move for many hours without incurring much metabolic stress.
I hope this helps.