Definition of "theca"

(biology) Any external case or sheath.

(botany) The pollen-producing organ usually found in pairs and forming an anther.  examples

(medicine) The twin layers of cells surrounding the basal lamina of an ovarian follicle.  examples

(microbiology, planktology) The membrane complex enveloping the cells of certain plankton including diatoms and dinoflagellates.

(marine biology) The calcareous wall of a corallite, the exoskeleton of a coral polyp.

(Christianity) A case for the corporal cloth used in the Eucharist.  examples