In baseball, it's the area enclosed by the 3 bases & home plate
The area of the field bounded by home plate and first, second, and ... 
Noun, 1. infield - the area of a baseball field that is enclosed by 3 bases and... [DOC] [TLE] infield - Memidex dictionary/thesaurus [PAR] "infield" definition: the area of a baseball field that is enclosed by 3 bases and 
home plate.