With apologies to John...
Ted Williams once said hitting a baseball is the hardest thing to do in sports and harder than flying a jet fighter in a war zone. Willie Mays said hitting makes no sense, trying to hit a round ball square with a round bat.
Hitting is the hardest tool to scout because no one really knows how a player will hit major league pitching until he's in the major leagues. We can be amazed at Joey Gallo's power or Brandon Nimmo's OBP and think the future is golden for them, but there's a fifty year historical draft precedent that says there's a 65% chance neither will ever make the majors.
When evaluating hitters, one has to evaluate the physical tools and skills he possesses. You cannot grade a player's hit tool by looking at a stat sheet. There's an old scouting term which goes something like, "Scouts scout tools, not performance" because performance, and their subsequent stats, are a result of his ability. Realistically, at the professional level scouts are scouting weaknesses, not strengths simply because they are easier to see.
The primary components you want to see when looking at a hitter are;
* A smooth, quick and level swing. We hear the terms "long" and "short" swing all the time, what defines swing length is how much the hands move from the start of the swing to impact. A player with a long swing will have excessive hand movement before and during the actual swing, what happens after impact is not a factor in measuring length. Ideally a hitter will start with his hands high, with the top hand no lower than the back shoulder. At the beginning of the swing, the lower body strides forward and the front hip starts to turn the body into the hitting position. If you watch a good hitter, it appears his lower body is swinging the bat, the rotation of the upper body in concert with the lower body "slots" the bat into the hitting zone. The only job of the hands during the swing, other than holding the bat, is to make minute adjustments to the pitch just before impact.
* Pitch Recognition. This is clearly a tool and hard to quantify because we can't crawl around in the player's brain to see what's he's thinking. Some guys have a freakish ability to wait until seemingly the last minute before committing to the swing, others have an 0-1 count on them in the on-deck circle. The best way to scout recognition is how a player contacts different pitches, some guys hit .380 on fastballs and couldn't hit secondaries with a boat oar, others have similar results regardless of pitch type.
* Controlling the strike zone. Just like recognition is the abiliity to hit different pitch types consistently, strike zone management is the ability to hit pitches in different parts of the strike zone. If a guy kills breaking balls away but can't touch them inside, then he's at a disadvantage over someone who can control the bottom of the zone.
* Ability to pull. The ability to pull the ball with authority is, in my opinion, unteachable. Hitters are constantly making adjustments, especially as they get older and their skills fade, to control the zone. A player who hits out of an open stance is only doing so for one reason; to compensate for a weakness. Luis Gonzalez and Andres Gallaraga became pull hitters later in their careers not because they all of a sudden learned to pull, but because they drastically altered their stances and approach to become pull hitters. With feet and hips open basically you're starting in a pre-impact position, from here it allows you to wait longer, gives the impression everything is inside, and all you do is flip the barrel into the zone and abracadabra, average to All-Star.
If I'm scouting a player during batting practice and a coach is grooving 80 mph meatballs to a right-handed hitter and he's hitting humpbacks to right center, I'm crossing him off my list. On the other hand, he's got the left-fielder running suicide drills and has the third baseman four feet in front of the outfield grass, I'm paying attention.
*Having a clue. Realistically, even at the major league level, there's a lot of guys who go to the plate with no clue of what they're going to do. How many times have we seen a pitcher struggling with his stuff and walk two straight guys on nine pitches only to have the next knucklehead hit into a double play on the first pitch? Stuff like that drives me crazy. The situation should dictate your game plan; pitcher, inning, score, runners, outs, things that matter in the continual flow of the game. Going to the plate and being a dummy interrupts that flow and costs your team scoring opportunities.
One thing I want to point out that for some reason is considered part of the hit tool when it is not is speed. I remember the infamous Byron Buxton/Mike Trout thread where more than one person said Buxton had a "7" hit tool because he'd get 30 infield hits a year because of his speed.
This is not true, on the contrary, getting jammed a lot or getting balls off the end of the bat could be viewed as a negative despite being fortunate to beat one out every once in awhile.
Hit tool is simply the ability to hit. It's not found in a box score or stat sheet or in an acronym or stopwatch, it's found at the ballpark.