Here are some of the terms I sling around here.

Adjusted ERA (ERA+): Calculated by Baseball Reference, this is the pitcher's ERA compared to the league ERA, adjusted for his ballpark. An ERA+ of 100 is exactly league average. Greater than 100 is better than average, less than 100 is worse. The advantage of ERA+ is that it equalizes ERA across eras and ballparks. For instance, Don Drysdale's 2.77 ERA in 1965--in a low-offense era (NL teams scored an average of 4.03 runs per game) in a pitcher's park--has an ERA+ of 118, the same as Chuck Finley's 4.17 ERA in 2000 (AL teams scored 5.30 runs per game, Jacobs Field favored hitters).

Batting Average (BA): You probably know this. Hits divided by at bats.

Earned Run Average (ERA): Another one you probably know. Earned runs per nine innings pitched, calculated (ER x 9) / IP.

On Base Percentage (OBP): (Hits + walks + hit by pitch) / (Hits + walks + hit by pitch + sacrifice flies). The advantage of on-base percentage compared to batting average is that it measures the frequency with which a batter gets on base (or a pitcher allows batters to get on base) rather than just the frequency of hits. 

On Base Plus Slugging (OPS): On-base percentage plus slugging percentage. A quick and easy way to combine the two measures.

Slash Line: A player's batting average, on base percentage, and slugging percentage, expressed as three figures separated by slashes. The major league average slash line in 2013 was .253/.318/.396.

Slugging Percentage (SLG): Total bases (i.e., one for a single, two for a double, three for a triple, four for a homer) divided by at bats. The advantage of slugging percentage compared to batting average is that it measures power. I read an essay from about 100 years ago (really) that said batting average is like asking someone how much change he or she has and answering "eight coins." Slugging percentage gives more weight to extra-base hits.

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