A single stem of a dandelion in its post-flowering state with the downy covering of its head intact. The term is applied when the flower is used, or is thought of as suitable for use, in a children's pastime by which the number of puffs needed to blow the filamentousachenes from a dandelion is supposed to tell the time.
1841 April 1, “Breakfast Table Science for Young People”, in Parley's Magazine, New York, page 112:
Dandelion clocks! -- Pray what are they? ... by far the best clock is made by blowing the downy seed of the dandelion.