If we’re talking about “expertise” instead of “experts,” it’s only natural to wonder what “expertise” actually is. Hold on to your butts, here comes the (light, I promise) conceptual analysis.
In simplest terms, expertise is just being good at something. That’s not the whole story, though — there are lots of ways to be good at something, but we don’t want to call all of them expertise.
Let’s look at basketball as a concrete example, and playing one-on-one, specifically. Being good at one-on-one basketball means something like “scoring more points than your opponent.” Obviously, lots of factors enter in to how good you are at this, including intrinsic ones (say, your height or your reflexes), extrinsic ones (how good your opponent is, whether the goal is regulation height), and acquired ones (your well-practiced jump shot, the tree-trunk legs you built from hours of squats in the gym). My position is that expertise is best identified with the acquired factors — and even more specifically, the mental ones (where “mental” is sort of hand-wavy for now, though I’m hoping to write more about that later).
If this definition holds generally, then expertise in a given domain is comprised of things that top performers in the domain have acquired. In Scrabble, this might include large lists of memorized words, concepts of “open” and “closed” boards and positions, statistics and probability around the distribution of available letters, and probably many more things that I, not a competitive Scrabble player, haven’t thought of.
These factors might be consciously acquired — learned — or they might not; it’s possible to develop a sense for the probabilities in late-game Scrabble without learning the statistics, but those word lists aren’t something that most people would memorize unintentionally. These factors, even the unconsciously-acquired ones, can be adjusted and replaced intentionally; look at how professional golfers break down and rebuild their swing (sometimes multiple times) from their childhood to the end of their career.
Thinking of expertise as the set of acquired factors that contribute to success in a domain also helps explain how there can be multiple paths to that success. Going back to basketball, if the goal is to score more points than the opponent, you can perform better in two general ways: you can optimize your offense (hoping that you can overwhelm your opponent with points), or you can optimize your defense (hoping that you can stop your opponent more often than you’re stopped). Either approach can help you achieve your goal, so either one is a valid path to being good at your domain.
Here’s one other way to explain this view of expertise: it’s the understanding you have of your domain as a system. You can understand a system to a greater or lesser extent; when you understand it more, you can make changes to it more safely, more predictably, with fewer side effects, etc.
We usually think about “understanding” in this sense as a set of consciously-accessible mental states (representations, models, whatever), but that needn’t be the case — “understanding” riding a bike, for instance, consists of subconscious, kinesthetic representations that integrate your sense of balance, where your body is in space, your awareness of the environment around you, etc.
The more detailed this understanding — the more expertise you have — the better (everything else being equal) you’ll be at whatever it is you’re doing.