Developmental delay is a concept within developmental sciences used to characterize instances where an individual's acquisition of skills or achievement of developmental milestones significantly lags behind age-appropriate expectations across one or more domains (e.g., cognitive, motor, language, social-emotional). As a research concept, it investigates the etiology, assessment, developmental trajectories, long-term outcomes, and intervention strategies associated with these deviations from typical development, serving as a critical framework for understanding normative processes and identifying atypical developmental patterns.