铁丸 (tiě wán) literally means 'iron pellet' — 铁 (tiě) means 'iron', and 丸 (wán) means 'pellet', 'ball', or 'pill'. Though it sounds like a generic metallic object, in modern Chinese usage, 铁丸 almost exclusively refers to the iron-rich pellets found in the gizzards of birds such as pigeons, chickens, and ducks. These pellets are swallowed intentionally by birds to aid digestion by grinding up hard food like seeds and grains in their muscular gizzard.

The term appears mainly in biological, veterinary, or agricultural contexts — for example, when discussing avian anatomy, poultry farming, or traditional Chinese veterinary practices. It is not used for man-made iron balls or industrial pellets; those would be called 铁球 (tiě qiú) or 金属颗粒 (jīnshǔ kēlì). 铁丸 carries a distinctly zoological nuance and is rarely encountered outside discussions about bird physiology or feeding behavior.