A carcinoma that arises from glandular epithelial cells of the esophagogastric junction. The disease is gastroesophageal junction adenocarcinoma (MONDO_0003219). Also known as: adenocarcinoma - GEJ, adenocarcinoma of cardioesophageal junction, adenocarcinoma of gastroesophageal junction, adenocarcinoma of the EG junction, adenocarcinoma of the GE junction, adenocarcinoma of the cardioesophageal junction, adenocarcinoma of the esophagogastric junction, adenocarcinoma of the gastroesophageal junction.