subfield
/ˈsʌbˌfiːld/Definitions
1. noun
A branch or specialized area within a particular field of study or activity.
“The researcher worked in the subfield of astrophysics.”
2. noun
A smaller area of land or territory within a larger field or district.
“The subfield was used for growing wheat.”
3. noun
A subgroup or subset within a larger category or classification.
“The subfield of mammals includes primates and carnivores.”