Malnutrition is a general term for a medical condition caused by an improper or insufficient diet. It most often refers to undernutrition resulting from inadequate consumption, poor absorption, or excessive loss of nutrients, but the term can also encompass overnutrition, resulting from overeating or excessive intake of specific nutrients. An individual will experience malnutrition if the appropriate amount of, or quality of nutrients comprising a healthy diet are not consumed for an extended period of time. An extended period of malnutrition can result in starvation, disease, and infection.

Malnutrition is the lack of sufficient nutrients to maintain healthy bodily functions and is typically associated with extreme poverty in economically developing countries. It is a common cause of reduced intelligence in parts of the world affected by famine. Malnutrition as the result of inappropriate dieting, overeating or the absence of a "balanced diet" is often observed in economically developed countries (eg. as indicated by increasing levels of obesity).

Most commonly, malnourished people either do not have enough calories in their diet, or are eating a diet that lacks protein, vitamins, or trace minerals. Medical problems arising from malnutrition are commonly referred to as deficiency diseases. Scurvy is a well-known and now rare form of malnutrition, in which the victim is deficient in vitamin C.

Common forms of malnutrition include protein-energy malnutrition (PEM) and micronutrient malnutrition. PEM refers to inadequate availability or absorption of energy and proteins in the body. Micronutrient malnutrition refers to inadequate availability of some essential nutrients such as vitamins and trace elements that are required by the body in small quantities. Micronutrient deficiencies lead to a variety of diseases and impair normal functioning of the body. Deficiency in micronutrients such as Vitamin A reduces the capacity of the body to resist diseases. Deficiency in iron, iodine and vitamin A is widely prevalent and represent a major public health challenge. An array of afflictions ranging from stunted growth, reduced intelligence and various cognitive abilities, reduced sociability, reduced leadership and assertiveness, reduced activity and energy, reduced muscle growth and strength, and poorer health overall are directly implicated to nutrient deficiencies. Also, another, although rare, effect of malnutrition is black spots appearing on the skin.

Hunger is the normal psychological response brought on by the physiological condition of needing food. Hunger can also affect the mental state of a person, and is often used as a metonym for general undernourishment.