Cumulative Record. Chapter 28: The Operational Analysis of Psychological Terms. Quote 4

Definition is a key term but is not rigorously defined. Bridgman’s original contention that the “concept is synonymous with the corresponding set of operations” cannot be taken literally, and no similarly explicit but satisfactory statement of the relation is available. Instead, a few roundabout expressions recur with rather tiresome regularity whenever this relation is mentioned. (p. 417)