tunicaPronunciation: /ˈtjuːnɪkə/Definition of tunica noun (plural tunicae /-kiː/) 1 Anatomy a membranous sheath enveloping or lining an organ: [with Latin modifier]:tunica media 2 Botany the outer layer or layers of cells in an apical meristem, which contribute to surface growth. Origin: late 17th century: from Latin, literally 'tunic'tunica in other Oxford dictionaries Definition of tunica in the US English dictionary