http://www.stathead.com/bbeng/woolner/statglossary.htm

Statistics Glossary
by Keith Woolner

Offensive Statistics

AVG (also BA) 
Batting Average -- hits divided by at-bats.  Probably the most common "rate"
statistic in baseball.  Has been historically used as the benchmark for great
hitters.  Advantages: commonly understood scale (everyone knows what at .300
hitter means), historically entrenched, reasonably easy to understand.
Disadvantages: neglects walks and extra-base hits.  Doesn't correlate with
offense as well as other measures.

OBP (also OBA) 
On Base Percentage -- Originally created by Branch Rickey and Allan Roth in
the 1950's as OBP = (H+BB+HBP) / (AB+BB+HBP).  The official stat, as adopted
in 1984, includes sacrifice flies in the denominator: OBP = (H+BB+HBP) /
(AB+BB+HBP+SF).  It is thus possible to have a lower OBP than AVG if your sac
flies exceed your walk total.  OBP is an incredibly important statistic,
since the one scarce resource in a baseball game in outs, and OBP measures
the rate at which outs are consumed.  League average OBP's have recently been
around .340, in contrast to the long-term average of .320-.330.

SLG (sometimes SA) 
Slugging Average or Slugging Percentage.  Defined as Total Bases per At-bat.
  Total bases are 1 for a single, 2 for a double, 3 for a triple, and 4 for a
home run (no extra credit is given for runners on base).  Thus, the formulae
for SLG are: SLG = TB/AB or SLG = (1B + 2*2B + 3*3B + 4*HR)/AB or SLG =
(H+2B+2*3B+3*HR)/AB.  League average SLG in the 90's has generally been
around .420, though earlier eras were more like .390-.400.

ISO 
Isolated Power: extra-bases per at-bat.  Invented by Rickey and Roth in the
1950's.   Extra bases are 0 for a single, 1 for a double, 2 for a triple, and
3 for a home run.  ISO = SLG - AVG, or ISO = (TB-H)/AB

TBP 
Total Base Percentage: Total Bases per Plate Appearance -- similar to SLG,
but uses the denominator from OBP (AB+BB+HBP) rather than just AB.  Not a
commonly used stat.

OPS 
Onbase Plus Slugging.  OPS = OBP + SLG.  Not a true rate statistic, since it
combines rate stats with different denominators, but it is a very common,
handy "index" to rank player production by.  Often, the decimal point is
omitted, thus a .380 OBP + .450 SLG = .830 OPS is written as simply 830 OPS.
 League average is roughly 750, top players will exceed 1000, while the worst
regulars are generally around 600.

PRO+ (sometime PRO or APRO) 
Adjusted Production.  A park- and league-adjusted version of OPS.  Used by
Total Baseball, and thus often quoted in comparing players from different
eras.   Defined as PRO+ = (OBP / LgOBP) + (SLG / LgSLG) -1, where OBP and SLG
have been adjusted for the player's home park, and LgOBP and LgSLG are the
league average OBP and SLG, respectively.  A PRO+ of 100 is league average
 (as in OPS, the decimal point is omitted).  Often thought of as the
"percentage" better than league average that a player's rate of production
was -- e.g. a PRO+ of 120 is "20% better" than league average, while a 95
PRO+ is "5% worse".  The percentages are not strictly true, but it's a useful
approximation.

ABR or BR/A (or sometimes just BR) 
Adjusted Batting Runs.  Part of Total Baseball's Linear Weights (LWTS)
system.   A measure of the number of runs batter generates beyond what an
average batter generates in the same number of plate appearances,
park-adjusted.  The formula is:
Runs = (.47)1B + (.78)2B + (1.09)3B + (1.40)HR + (.33)(BB + HB) - (.25)(AB-H)
- (.50)OOB
The -0.25 value for an out is modified depending on the league and season in
question such that the league ABR total is zero (thus an ABR of zero means
the batter was exactly average).

RC 
Runs Created -- Bill James's model for offensive production.  In its most
basic form, RC = OBP*TB.  The intuitive explanation is that scoring runs
consists primarily of two actions: getting on base or creating baserunners
(the OBP term), and power or advancing runners around the bases (the TB
component).  Note that since SLG = TB/AB, RC can also be written as RC =
OBP*SLG*AB, which is the justification for the use of OPS as a useful index.
 There are a variety of more technical versions that include stolen bases,
double plays, and correct for the lack of complete data in years past.   Runs
Created is most accurate for teams, but is often used to measure individual
production.  It has a tendency to overrate top offensive players, who get
"double-credit" for "driving themselves in" when combining their own OBP and
SLG.

RC/25 and RC/27 
Runs Created per 25 (27) Outs -- the rate statistics associated with Runs
Created.   It is derived by simply taking the total Runs Creatd, and dividing
it by the number of outs the player made (AB-H, sometimes adding CS or GIDP
depending on the available data), and multiplying by 27 outs. The resulting
figure is an estimate of how many runs a team made up of 9 clones of the
player would score per game.  Sometimes 25 outs is used instead of 27 since
there are some outs (such as baserunning outs) that do not appear in the
batting line.  Approximately 25 outs per game (of 27 total for a full 9
inning game) are accounted for by the traditional batting stats on average,
hence the popularity of RC/25.

MLV and MLVr 
Marginal Lineup Value (and rate) -- based on Runs Created, MLV corrects the
shortcomings using of RC for individual players by estimated how many
additional runs an average team scores if you replace an average batter with
the individual.  This eliminates the "batting yourself in" flaw in individual
RC, and correctly estimates the impact of the extra team plate appearances a
player creates with a high OBP.  A full explanation is available in the
Stathead Baseball Engineering Library at http://www.stathead.com.  MLV is the
total number of runs a player adds over the number of games he plays over the
course of a season, while MLVr is a rate stat, and measures MLV per game.

SBR 
Stolen Base Runs  SBR = .3*SB-.6*CS or SBR = .3*(SB-2*CS).  Total Baseball's
approach to quantifiying base-stealing.  Numerous statistical studies show
that the breakeven success rate for steals (the rate at which attempting to
steal is neither helping nor hurting the team in terms of total runs scored)
is about 67% -- below that you are costing your team runs.  Each successful
steal adds about .3 runs to a team's total -- far less than in generally
believed.  SBR estimates the impact of base-stealers, which, other than the
elite base-stealers, rarely amounts to more than a few runs per year.

MLE 
Major League Equivalency -- Bill James/STATS method for translating minor
league performance to what the such player would have hit had he been in the
majors.  MLE's normalize past performance, rather than predicting future
performance.  The method for MLE's has never been fully published, though
James describes a precursor to the current approach in his 1985 Baseball
Abstract.  It involves making an "league difficulty" adjustment to a player's
stats to a standard major league level similar to how park-adjustments are
done to normalize a player's performance to a neutral park.

EqA (aka Davenport Translations)
Equivalent Average -- Invented by Clay Davenport, and published in the
Baseball Prospectus, EqA combines an alternate system to MLE for translating
minor league performance with a normalizing step that converts translated
performance to a single number on the same scale as batting average.  E.g. a
minor league player may hit .300/.400/.500 in the AAA Pacific Coast League.
 Given that the PCL is a high-offense league, that might translate to a
.265/.330/.440 major league equivalent performance, which in turn is
summarized as a .270 EqA, which is slightly above the average major league
performance (set at .265).  EqA has an advantage over MLE's in that EqA's are
computed down to A-ball, and thus can be used to track players throughout the
minors.   There is also somewhat more information published on the EqA method
in the Baseball Prospectus than on the James MLE method in the STATS
Handbooks.