A lamella (plural lamellae) is a small plate or flake, from the Latin, and may also be used to refer to collections of fine sheets of material held adjacent to one another, in a gill-shaped structure, often with fluid in between though sometimes simply a set of 'welded' plates. The term is used in biological and engineering contexts, such as filters and heat exchangers. The microscopic structures in bone and nacre are lamellae in the materials science sense of the word.