from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. A glycoside, related to salicin, found in the bark of certain species of the poplar (Populus).
from the GNU version of the Collaborative International Dictionary of English
- n. A glycoside, related to salicin, found in the bark of certain species of the poplar (Populus), and extracted as a sweet white crystalline substance.
from The Century Dictionary and Cyclopedia
- n. A crystallizable substance (C20H22O8) found in the bark, root, and leaves of the aspen, Populus Tremula, along with salicin. It forms delicate white needles, which have a sweet taste like that of licorice.
Latin populus poplar + -in: compare French populine. (Wiktionary)
Aspen contains salicin and populin, which have properties similar to aspirin in reducing fever, pain and inflammation.
In the willow and poplar,  a crystalline, bitter substance, salicin or populin, is found.