Undernutrition is a state resulting from insufficient intake or absorption of nutrients, leading to a deficit in energy and/or essential micronutrients required for healthy growth, development, and maintenance. As a critical concept within public health, nutrition science, and global development research, it investigates the complex aetiology, physiological impacts, epidemiological patterns, measurement methodologies, and effective prevention and treatment strategies for various forms of nutrient deficiency and energy deficit across diverse populations and life stages, underscoring its significant global health burden and socio-economic implications.