Cleistogamic refers to a plant's ability to self-pollinate without opening its flowers. There aren't many synonyms for this specific term, but some related words include autogamous, self-fertilizing, and self-pollinating. These words all describe the same process of plants being able to reproduce without relying on pollinators. Another term that can be used in a similar context is apomixis, which refers to the formation of seeds without fertilization. While these terms are not direct synonyms for cleistogamic, they can be used in similar contexts to describe methods of plant reproduction that do not rely on external pollinators.