retroperitoneum

noun

ret·​ro·​per·​i·​to·​ne·​um -ˌper-ət-ᵊn-ˈē-əm 

plural retroperitoneums or retroperitonea -ˈē-ə 

: 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