gizzard

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A modified muscular pouch behind the stomach in the alimentary canal of birds, having a thick lining and often containing ingested grit that aids in the breakdown of seeds before digestion.
2. n. A similar digestive organ found in certain invertebrates, such as the earthworm.

Century Dictionary and Cyclopedia

1. n. The second stomach of a bird, not counting the crop or craw as the first; the bulbous or muscular stomach (ventriculus bulbosus), succeeding the proventriculus and succeeded by the duodenum; the gigerium. In most birds, especially those which feed upon grain or hard seeds, it is very thick and muscular, and lined with tough leathery (or even bony) epithelium, the organ thus forming a powerful grinding-mill in which the food is triturated after being mixed with the gastric juice of the preceding glandular stomach.
2. n. The proventriculus or first true stomach of insects, generally armed inside with horny teeth. See cut under Blattidæ.—
3. n. The stomach of some mollusks, as Bullidæ, when muscular and hardened.—
4. n. Figuratively, temper: now only in the phrase to fret one's gizzard.

Wiktionary

1. n. A portion of the esophagus of either a bird or an annelid that contains ingested grit and is used to grind up ingested food before it is transferred to the stomach.

GNU Webster's 1913

1. n. (Anat.) The second, or true, muscular stomach of birds, in which the food is crushed and ground, after being softened in the glandular stomach (crop), or lower part of the esophagus; the gigerium.
2. n. A thick muscular stomach found in many invertebrate animals.
3. n. A stomach armed with chitinous or shelly plates or teeth, as in certain insects and mollusks.

WordNet 3.0

1. n. thick-walled muscular pouch below the crop in many birds and reptiles for grinding food