A vascular leaf that is part of an heteroblastic series, and is characterized by anatomical features that are intermediate between juvenile and adult leaves. [source: GR:Chih-Wei_Tung]
Many species have juvenile leaves at the base of the stem, adult leaves at the apex, and transition leaves in between. In maize, juvenile leaves are at the tip of the stalk (stem), which differentiates first, with adult leaves at the base, while other grasses may have other arrangements. Transition leaves may have mosaics of juvenile and adult tissues, as in Brachypodium.