Malnutrition is eating a diet which does not supply enough nutrients. There are two broad cateogries of malnutrition:
Sometimes, people need to lose weight, and eat according to a special diet. This is usually not called malnutrition.
Malnutrition is considered a more scientific and official term than starvation which has negative associations. Malnutrition is not, however, simply a matter of eating too little. Someone can be malnourished if they eat an unhealthy diet. This can be cured by eating a proper balanced diet.