1. Any thin, flat, platelike or sheetlike structure.
2. The thin and usually flattened blade of a leaf, in which photosynthesis and transpiration occurs. The bulk of the lamina is made up of mesophyll cells interspersed by a network of veins (vascular bundles). The mesophyll is enclosed by a protective epidermis that produces a waxy cuticle.
3. The leaflike part of the thallus of certain algae, notably kelps. See also stipe.