Noun. 1. duodenum-the part of the small intestine between the stomach and the jejunum. small intestine-the longest part of the alimentary canal; where digestion is completed. Translations. [ˌdjuːəʊˈdiːnəm] N (duodenums or duodena (pl)) [ˌdjuːəʊˈdiːnə] → duodeno m.