the silky fibers around the seeds of any of several silk-cotton trees, esp. a ceiba (Ceiba pentandra): used for stuffing mattresses, life preservers, sleeping bags, etc.
See kapok in American Heritage Dictionary 4
(kāˈpŏkˌ)
noun
A silky fiber obtained from the fruit of the silk-cotton tree and used for insulation and as padding in pillows, mattresses, and life preservers.