Small Bowel Obstruction: This is the most likely diagnosis because a fluid-filled terminal ileum is a common finding in small bowel obstruction, where the ileum becomes dilated with fluid due to a mechanical blockage preventing normal flow of intestinal contents.

Ileus: Postoperative or adynamic ileus can cause a fluid-filled terminal ileum due to decreased motility of the bowel, leading to accumulation of fluid.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease can cause inflammation and narrowing of the ileum, potentially leading to a fluid-filled appearance on CECT due to obstruction or decreased motility.
Infection: Infectious ileitis, such as that caused by Salmonella or Yersinia, can lead to inflammation and fluid accumulation in the terminal ileum.

Intussusception: Although less common in adults, intussusception can cause a fluid-filled terminal ileum and is a critical diagnosis not to miss due to the risk of bowel ischemia and necrosis.
Volvulus: A volvulus of the small bowel can cause obstruction and a fluid-filled appearance of the terminal ileum, requiring prompt diagnosis and treatment to prevent ischemia.
Ischemic Bowel: Ischemia of the bowel can lead to a fluid-filled terminal ileum due to decreased motility and inflammation, and is a critical diagnosis due to the high risk of bowel necrosis and perforation.

Lymphoma: Although rare, lymphoma involving the terminal ileum can cause obstruction and a fluid-filled appearance on CECT.
Carcinoid Tumor: A carcinoid tumor in the terminal ileum can cause obstruction and fluid accumulation, and is an important diagnosis to consider due to its potential for malignancy.
Meckel's Diverticulum: A complication of Meckel's diverticulum, such as obstruction or inflammation, can lead to a fluid-filled terminal ileum, although this is a rare cause.