The membrane stacks within a chloroplast are called grana.
A granum is a stack of flattened membrane sacs called thylakoids; grana is the plural form. These stacks are embedded in the chloroplast stroma and contain much of the machinery for the light-dependent reactions of photosynthesis, including chlorophyll and photosystem II.
Grana are connected by unstacked membrane regions called stroma lamellae, or intergranal thylakoids. This arrangement creates one interconnected thylakoid membrane system rather than a collection of isolated stacks. The stacked design provides extensive membrane surface for light-harvesting and electron-transfer components.
A common mix-up is reversing the terms: thylakoids are the individual sacs, while grana are the stacks made from them. The thylakoid lumen is the internal space inside each sac, whereas the surrounding fluid is the stroma. In many illustrations, grana look like piles of coins, which is a useful—if simplified—visual model.