flower bud

蕾 is composed of the radical of 艸 (grass) and 雷 (thunder). 艸 conveys the idea of plants or vegetation, while 雷 conveys the tremendous booming sound that follows lightning. Generally, thunder is followed by rain, thus 蕾 conveys the idea of the state of a flower before it fully blooms (like buds waiting for the nourishing rain after thunder), means flower bud.