Vitamin D deficiency is a widespread global public health concern affecting more than one billion individuals worldwide. Traditionally recognized for its role in calcium homeostasis and bone metabolism, vitamin D is now known to influence multiple physiological systems, including immune regulation, cardiovascular health, endocrine function, and neuromuscular performance. Deficiency in adults has been associated with osteoporosis, osteomalacia, increased susceptibility to infections, metabolic disorders, cardiovascular diseases, autoimmune conditions, and adverse mental health outcomes. This review examines the clinical significance of vitamin D deficiency in adults, focusing on epidemiology, physiological functions, risk factors, clinical manifestations, diagnostic approaches, and therapeutic interventions. Emerging evidence suggests that maintaining optimal vitamin D status may contribute to improved overall health and disease prevention. Early detection and appropriate supplementation remain critical strategies in reducing the burden of vitamin D deficiency globally.