a flattened sac or vesicle lined with a pigmented membrane that is the site of photosynthesis, in plants and algae occurring in interconnected stacks constituting a granum of the chloroplast, and in other photosynthesizing organisms occurring either singly or as part of the cell membrane or other structure.
Inside it is an empty area called the thylakoid space or lumen.
thylakoid in Science
(thī'lə-koid') A saclike membrane that contains the chlorophyll in cyanobacteria and in the chloroplasts of plant cells and green algae. In chloroplasts, thylakoids are arranged in stacks called grana.