What is the difference between epithelium and epidermis?
The Epidermis is a type of epithelium tissue.
Epithelium a type of tissue (the others being Muscle, Connective and Nervous) that lines internal and external surfaces. They are typically thin sheets of cells that have a range of functions such as: separating parts of the body, lining organs (like the lungs) and generally protecting your body (e.g. The skin - including the epidermis - protects the inside of your body from external dangers).
The Epidermis is the outermost, protective layer of skin, consisting of 4 other layers in thin skin (e.g. on eyelids) and 5 on thick skin (e.g. soles of feet). It is a type of epithelial tissue and is responsible for primarily protection but also plays a part in temperature regulation through perspiration (sweating).
"Epidermis" can also have connotations of plant tissue. In this context, epidermis refers to the outer layer of cells covering the exterior areas of a plant (stem, leaves, flowers and roots). This helps to protect against water loss, it provides a physical barrier against pathogens and pests and it aids gas transfer and water/mineral ion absorption in roots. The epidermis in a plant is often transparent as well so that light can travel through to the lower layers of plant tissue for photosynthesis.
(Thank you Indrani R for your help with this!)
Hope this helps! Let me know if you need anything adding.