The soft white fluff attached to the seeds of a thistle. When the flower dies, each seed grows a little parachute of fine hairs, the wind takes it, and the seed travels far away — that is how the plant spreads. It is an uncountable noun: 'thistledown', not 'a thistledown', not 'thistledowns'. Say 'a piece of thistledown' if you need one. You will hardly ever hear this word in ordinary talk. It lives in two places. One is nature writing: 'thistledown blowing across the field'. The other, more useful to you, is comparison — it is the standard English picture for something very light, very soft, and carried helplessly by the wind: 'light as thistledown', 'she drifted like thistledown', 'hair as fine as thistledown'. That is really why the word survives. If you have seen the white puffball of a dandelion that children blow — 蒲公英 — thistledown is the same idea from a different plant, and English speakers often mean that sort of thing loosely when they say it.

薊花冠毛、薊絮（薊的種子上那層會隨風飄的白色細絨毛）。想像蒲公英那顆白色絨球被吹散的樣子，就是這個。英文常用它比喻「輕飄飄」。