The term "tunica" refers to a layer or covering in biological and anatomical contexts. It is often used to describe the various layers of tissue that surround organs or structures in the body. For example, the tunica intima is the innermost layer of blood vessels, while the tunica media is the middle layer that contains smooth muscle. In botany, "tunica" can also refer to a protective layer surrounding certain plant structures, such as bulbs. The tunica helps to protect the inner parts of the plant and can play a role in growth and development. This dual usage highlights the importance of tunica in both animal and plant biology.