The silky down attached to the seedlike fruit of a thistle; pappus.(noun)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "thistledown" in a sentence
"A thing as important as thistledown is as unimportantly dismissed."
"A thin mist floated like thistledown from the marshes, which were so distant that they were visible only as a pinkish edge to the horizon."
"We also believed that there's a secret name which, if you call the thistledown by, will make it fly into your hand."