From BR Bullpen

The infielders are the defensive players who are positioned roughly on the lines between first, second, and third base. From right to left (as seen from home plate) they are designated the first baseman, second baseman, shortstop, and third baseman. Although the pitcher and catcher also play in the infield, they are not normally classified as infielders.

The first baseman and third basemen are called corner infielders because they are positioned near the corners of the infield diamond, while the second baseman and shortstop are called middle infielders because they are positioned near the middle of the field. The defensive demands of first base are very different from the other three infield positions, so people will sometimes use the term as though it excludes first basemen.

  • [1] Article on power-hitting middle infielders in The Hardball Times.
Baseball positions
Outfielders: Positions.png Left field | Center field | Right field
Infielders: 3rd base | Shortstop | 2nd base | 1st base

Pitcher | Catcher

Designated hitter