NCI: A malignant tumor with glandular differentiation arising predominantly from Barrett mucosa in the lower third of the esophagus. Rare examples of esophageal adenocarcinoma deriving from ectopic gastric mucosa in the upper esophagus have also been reported. Grossly, esophageal adenocarcinomas are similar to esophageal squamous cell carcinomas. Microscopically, adenocarcinomas arising in the setting of Barrett esophagus are typically papillary and/or tubular. The prognosis is poor.