theca
noun
the·ca ˈthē-kə 
plural thecae ˈthē-ˌsē
-ˌkē
: an enveloping sheath or case of an animal or animal part
thecal
ˈthē-kəl
adjective 
Word History
Etymology
New Latin, from Greek thēkē case — more at tick entry 4
First Known Use
circa 1666, in the meaning defined above
Medical Definition
theca
noun
the·ca ˈthē-kə
plural thecae ˈthē-ˌsē -ˌkē
-ˌkē : an enveloping case or sheath (as the theca folliculi) of an anatomical part