Fred Kerlinger's definitions

Scientific research is a systematic, controlled, empirical, and critical investigation of hypothetical propositions about the presumed relations among natural phenomena

Theory: a set of interrelated constructs (concepts), definitions, and propositions that present a systematic view of phenomena by specifying relations among variables, with the purpose of explaining and predicting the phenomena.

Hypothesis: a tentative proposition about the relation between two or more phenomena or variables. 

Concept: Expresses an abstraction, e.g., "achievement."

Construct: a concept which is invented or adopted for a special scientific purpose, e.g., "intelligence."

Constitutive definition: defines a construct with other constructs, e.g. "weight" by saying that it is "heaviness."

Operational definition: assigns meaning to a construct or a variable by specifying the activities or "operations" necessary to measure it.