rec·tum 
(rĕktəm)
(rĕktəm)n. pl. rec·tums or rec·ta (-tə)
The terminal portion of the large intestine, extending from the sigmoid colon to the anal canal.
[Middle English, from Latin (intestīnum) rēctum, straight (intestine), neuter of rēctus; see reg- in the Appendix of Indo-European roots.]