A part of an organ that constitutes a distinct modular sub-unit. In some cases, the organ may also contain other sub-units of identical or similar types, in other cases this may be a distinct entity.

FMA distinguishes segment from zone by whether the fiat boundaries are fixed/anchored (segments) or floating (zone). It's not completely clear how to apply this distinction

Organ region with one or more anchored fiat boundaries. Examples: artery, trunk of nerve, cervical part of esophagus, pelvic part of vagina, horn of thyroid cartilage, anterior segment of eyeball., Organ region with one or more fixed or anchored fiat boundaries. Examples: artery, trunk of nerve, cervical part of esophagus, pelvic part of vagina, horn of thyroid cartilage, anterior segment of eyebal.

uberon

organ segment, organ region with fixed fiat boundary, segment of organ

UBERON:0000063

http://purl.obolibrary.org/obo/uberon.owl, http://purl.obolibrary.org/obo/cl.owl