plastid Syllabification: (plas·tid)Pronunciation: /ˈplastid/Definition of plastid noun Botany any of a class of small organelles, such as chloroplasts, in the cytoplasm of plant cells, containing pigment or food. Origin: late 19th century: from German, based on Greek plastos 'shaped'plastid in other Oxford dictionaries Definition of plastid in the British & World English dictionary