To undernourish means to fail to provide enough food or the right kinds of food for proper health and growth. When a plant is undernourished, it might have yellow leaves and weak stems because it lacks nutrients from the soil. When a person is undernourished, their body doesn't get enough calories, protein, vitamins, or minerals to function well.

Undernourishment is different from simply feeling hungry before dinner. It means going without adequate food over weeks or months, which affects how the body works and grows. An undernourished child might have less energy, get sick more easily, or grow more slowly than they should. In some parts of the world, people face undernourishment because they lack access to enough nutritious food.

The word can also be used more broadly. You might say that a student who only reads comic books and never touches other literature is intellectually undernourished, meaning their mind isn't getting the variety it needs to develop fully. A garden undernourished by poor soil produces weak, scraggly plants instead of strong, healthy ones.

The related noun is undernourishment or undernutrition, which describes the condition of being undernourished. People and organizations work to address undernourishment through food programs, farming improvements, and nutrition education.