Theory refers to an organized set of concepts that explains a phenomenon or set of phenomena.
Other /More definition:
Theory is a term in Human development that refers to an organized set of ideas that is designed to explain (human) development. There are five (5) general perpectives that influence current research, although in truth there are comprehensive theories of human development to guide research:
1. Psychodynamic theory
2. Learning theory
3. Cognitive theory
4. Ecological and systems theory
5. Theories involving the life-span perspective, selective optimization with compensation, and the life-course perspective.
List of books: Theory