Noun

1. Technical matters concerning baseball not apparent to spectators.
   • 1914, Paul Withington and Lothrop Withington, The Book of Athletics, Lothrop, Lee & Shepard,

     There always has been a tendency to overlook the catcher, possibly because so much of him is covered up during the game, but more because the greatest part of his work is inside baseball and of the kind which the general public can neither see nor appreciate.

   • 1993, Michael A. Katovich, "Humor in Baseball: Functions and Dysfunctions", in The Journal of American Culture, volume 16, issue 2, pages 7–16,

     Here, the game is marked by the "little things" that have come to be defined as "inside baseball".

2. Matters of interest only to insiders.
   • 2003, Sam Hirsch, "The United States House of Unrepresentatives: What Went Wrong in the Latest Round of Congressional Redistricting", in Election Law Journal, volume 2, issue 2, pages 179–216,

     But in fact, redistricting remained very much a game of inside baseball.