Phyllode

Definition
noun, plural: phyllodes
(1) A modified petiole in some plants in which the petiole is characteristically flattened resembling and performing functions similar to a true leaf, even replacing the true leaves as major photosynthetic structure in certain plant groups.
(2) (bryology) The leaf-like, green structure in moss plants involved in light capturing, photosynthesis, and water absorption.
Supplement
In flowering plants, a petiole is generally slender and chiefly renders support by attaching the leaf blade to the stem. In some plants, the petioles become modified or specialized in a way that they become leaf-like in appearance (expanded) and serve not only to support the leaf but to carry out photosynthesis as well. An example is the acacia. The phyllodes of Acacia koa for instance are thick and coriaceous helping the plant in surviving stressful environment.
In mosses, the phyllodes refer to the leaf-like structures of the moss plant in the gametophyte stage. The phyllodes are not regarded as true leaves although similar in appearance and function. It is because phyllodes do not have vascular tissues.
Word origin: from New Latin phyllodium, from Greek phullōdēs (leaflike)