The antonyms for the term "overfilled" are "underfilled," "empty," and "half-filled." Underfilled refers to a situation where the container or space is not filled to capacity, whereas empty refers to having nothing inside the container or space. Half-filled, on the other hand, means that the container or space is filled only halfway. Overfilling can lead to spillage, mess, and reduced capacity for further storage or use. In contrast, underfilling or maintaining space within a container can facilitate proper storage, easy handling, and possibly save on the cost of material, weight, and space.