retroperitoneum
noun
ret·ro·per·i·to·ne·um
| \ -ˌper-ət-ᵊn-ˈē-əm
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plural retroperitoneums
or retroperitonea\ -ˈē-ə
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Medical Definition of retroperitoneum
: the space between the peritoneum and the posterior abdominal wall that contains especially the kidneys and associated structures, the pancreas, and part of the aorta and inferior vena cava