cecum - the cavity in which the large intestine begins and into which the ileum opens; the appendix is an offshoot of the cecum. blind gut, caecum. bodily cavity, cavum, cavity - (anatomy) a natural hollow or sinus within the body. large intestine - beginning with the cecum and ending with the rectum; includes the cecum and the colon and the rectum; extracts moisture from food residues which are later excreted as feces.