Pretend you knew all along
Adjusted Earned Run Average (ERA+)
ERA+ takes a player's ERA and normalizes it across the entire league. It accounts for external factors like ballparks and opponents. It then adjusts, so a score of 100 is league average, and 150 is 50 percent better than the league average.
At Bat (AB)
A plate appearance that doesn’t end in a sacrifice fly, walk, or hit by pitch.
Base on Balls (BB)
Base on balls, otherwise known as walks.
Batting Average (BA or AVG)
Hits divided by at bats. A measurement of how often a player gets a hit.
Exit Velocity (EV)
Similar to launch angle, this is information that became public after the 2015 season for the first time. It is the measure of the speed of the ball leaving the bat after contact. Higher velocities are better, but it’s combined with launch angle that they find the best exit velocity for home runs in combination with the ideal launch angle is 100-110 MPH.
Expected Batting Average (xBA)
Expected Batting Average (xBA) is a Statcast metric that measures the likelihood that a batted ball will become a hit. Each batted ball is assigned an xBA based on how often comparable balls -- in terms of exit velocity, launch angle and, on certain types of batted balls, Sprint Speed -- have become hits since Statcast was implemented Major League wide in 2015. (As of January 2019, xBA now factors in a batter's seasonal Sprint Speed on "topped" or weakly hit" balls).
Expected Earned Run Avg (xERA)
xERA is a simple 1:1 translation of xwOBA, converted to the ERA scale
Expected Weighted On-base Average (xwOBA)
xwOBA is formulated using exit velocity, launch angle and, on certain types of batted balls, Sprint Speed.
Fielding Independent Pitching (FIP)
FIP is similar to ERA, but it focuses solely on the events a pitcher has the most control over -- strikeouts, unintentional walks, hit-by-pitches and home runs. It entirely removes results on balls hit into the field of play.
Hit (H)
A ball put in play in fair territory with no out recorded that a fielder would not have “normally” have been expected to catch.
Home Run (HR)
A four-base hit. Lots of nicknames for the home run, including homer, bomb, and many others.
Launch Angle
The angle that a ball leaves the bat. Too shallow, and the ball is always a ground ball. Too high, and it’s a pop up. Per research from 2015 data, the best angle for a home run is roughly 20-30 degrees, though on average, Kris Bryant led the majors last year with just over a 19-degree average launch angle.
On Base Percentage (OBP)
The times a batter has been on base, calculated by walks plus hits plus hit-by-pitch, then dividing that by at bats plus walks plus hit-by-pitch plus sacrifice flies.
On Base Plus Slugging (OPS)
A combination measure of adding on base percentage and slugging percentage together as a measure of total offensive contribution.
On Base Plus Slugging Plus (OPS+)
A weighted statistic for OPS by comparing the OPS of every hitter in a particular stadium and how an individual hitter performs in comparison. The score is 100 for exactly league average, and above 100 indicates better than league average, below 100 indicates worse than league average.
Outs Above Average (OAA)
A range-based metric of skill that shows how many outs a player has saved over his peers.
Plate Appearance (PA)
Any time a player completes a time to the plate (Unlike at-bats, this metric takes into consideration sacrifice flies, walks, and hit by pitch). Used in certain percentage calculations.
Run (R)
When a player scores a run.
Runs Batted In (RBI)
When a batter does something that causes a run to score, other than by error, he is awarded a run batted in.
Runs Created (RC)
A statistic to estimate the number of runs that a hitter contributes to his team. There are multiple ways to compute this, much like wins above replacement (WAR), so watch the scoring, but the original formula from Bill James was walks plus hits, then taking that number times total bases, then dividing that amount by the sum of at bats plus walks.
Sacrifice Fly (SF)
When a player hits a fly ball and the ball is caught, but a runner scores on the play. This does not count as an at-bat, rather only a plate appearance.
Slugging Percentage (SLG)
A measure of the amount of bases that each hit a player gets. Rather than using hits like batting average, it takes total bases divided by at bats.
Stolen Base (SB)
When a runner goes from one base to another during a pitch.
Walks And Hits Per Inning Pitched (WHIP)
The statistic shows how well a pitcher has kept runners off the basepaths, one of his main goals. The formula is simple enough -- it's the sum of a pitcher's walks and hits, divided by his total innings pitched.
Weighted Runs Created (wRC)
Statistic utilizing weighted on base average (wOBA, utilized to attempt to quantify the contribution that each hit plays rather than weighing each kind of hit or walk equally) and finding how that compares to the league overall. The more prominent statistic is wRC+, which is a league- and park-adjusted version of this statistic.
Wins Above Replacement (WAR)
A number that attempts to place a value on the number of wins that a player provides his team above the replacement-level player in the league. There are seemingly different formulas for each different WAR. Fangraphs and Baseball-Reference each add in their own formulas, and when sourced in writing, you often see them abbreviated as fWAR for Fangraphs WAR and bWAR for Baseball-Reference’s WAR. The major differences in the calculations have to do with how each uses defense, whether a simple positional adjustment, using a metric measuring range against the rest of the league, or using something like defensive runs saved to weigh as the defensive factor in the calculation.