Botany: photosynthetically active parenchyma tissue mainly occurring in leaves. Often but not always it is differentiated into two types:

Palisade mesophyll: The tissue layer(s) of a leaf just below the leaf epidermis. Most important for photosynthesis. Usually the cells are arranged densely and leave no space between them. Chloroplasts are particularly concentrated. Most selective agromyzid leaf miners are feeding on this tissue.
Spongy mesophyll: Mesophyll tissue below the palisade mesophyll consisting of irregularly shaped cells with much air space between them.

Alternative forms for mesophyll : palisade mesophyll, spongy mesophyll.