[thahy-luh-koid] 
| 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. |
s resembling a bag, equiv. to thȳ́lak(os) sack + -oēidēs -oid 
| thy·la·koid (thī'lə-koid') n. A saclike membrane that is the structural unit of the grana in the chloroplasts of plant cells. [Greek thūlakos, sack + -oid.] |
| thylakoid (thī'lə-koid') Pronunciation Key
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. |