Grana (singular, granum):

Part of the internal structures within chloroplasts, seen as green granules with the light microscope and as a series of parallel lamellae with the electron microscope. It is consisting of 5 - 30 membranous disc (thylokoids), 0.25 - 0.8 µm in diameter. There may be 40 - 80 grana in a typical chloroplast. (Allaby, 1998)