involucrum
noun
in·vo·lu·crum ˌin-və-ˈlü-krəm 
plural involucra ˌin-və-ˈlü-krə
: a surrounding envelope or sheath
Word History
Etymology
borrowed from New Latin involūcrum, going back to Latin, "wrapper, cover, envelope," from involū-, variant stem of involvere "to move by rolling, roll back on itself, enclose in a covering, wrap up" + -crum, variant of -culum, instrumental suffix (going back to Indo-European *-tlom) before a preceding l — more at involve
First Known Use
1657, in the meaning defined above
Medical Definition
involucrum
noun
in·vo·lu·crum ˌin-və-ˈlü-krəm
plural involucra -krə
: a formation of new bone about a sequestrum (as in osteomyelitis)