Underweight describes someone who weighs less than is healthy for their height and age, or something that weighs less than it should. When doctors say a child is underweight, they mean that child may need to gain weight to be healthy and strong. Just as being overweight can cause health problems, being underweight can too: it might mean someone isn't getting enough nutrition to grow properly, fight off illnesses, or have energy for school and activities.

The word can also describe objects or shipments that weigh less than expected. If a delivery truck arrives with an underweight load, the cargo weighs less than what the shipping documents said it should.

Underweight is different from simply being thin or small. Some people are naturally slim and perfectly healthy. Underweight specifically means falling below the healthy weight range for someone's body, which doctors determine using growth charts and other measurements. A person who's underweight might need more nutritious foods or medical care to find out if something is preventing them from maintaining a healthy weight.