Focus is the continuous, iterative process of keeping the main thing the main thing.
The verb and noun forms of “Focus” mean different things. For simplicity, I’m going to call the the noun form “intensity.” So, intensity is the ability to be 100% engaged in what you are doing at any given time.
Focus and intensity are analogous to effectiveness and efficiency or leadership and management. The former is about doing the right things and the latter is about doing things right.
As a result, the whole point of focus is to ensure that we’re optimizing for the entire-ity of the main thing or goal or how we will measure our life. It is easier to understand this with a manufacturing analogy. It is useless for a car manufacturer to produce more doors than a chassis requires. Similarly, it is pointless to optimize on one part of our goal (say, our career) if all else is suffering. Making progress on the main thing in its entire-ity is productivity. Everything else is activity.
So, focus is how we stay productive. That’s exactly why the process of doing so is continuous and iterative – it is hard to stay productive as life happens.
Finally, it is helpful to think of focus as we think of balance in our lives. We are always in the act of balancing, never completely balanced. Focus is no different.