Relationships between a number of objects or events and thus not much different from a concept. However, a hypothetical construct is where a process is not directly observable or objectively measurable, but is assumed to exist because it gives rise to measurable phenomena. In the past, Karl Pearson (1857-1936) suggested it as a substitute for concept.
See Composite (or sum) scale, Concept, Construct equivalence, Construct validity, Convergent validity