Source : Webster's Revised Unabridged Dictionary (1913)

Poplar \Pop"lar\, n. [OE. popler, OF. poplier, F. peuplier, fr.
   L. populus poplar.] (Bot.)
   1. Any tree of the genus {Populus}; also, the timber, which
      is soft, and capable of many uses.

   Note: The aspen poplar is {Populus tremula} and {P.
         tremuloides}; Balsam poplar is {P. balsamifera};
         Lombardy poplar ({P. dilatata}) is a tall, spiry tree;
         white poplar is {Populus alba}.

   2. The timber of the tulip tree; -- called also {white
      poplar}. [U.S.]

Aspen \Asp"en\ ([a^]s"p[e^]n), Asp \Asp\ ([.a]sp), n. [AS.
   [ae]sp, [ae]ps; akin to OHG. aspa, Icel. ["o]sp, Dan. [ae]sp,
   Sw. asp, D. esp, G. espe, ["a]spe, aspe; cf. Lettish apsa,
   Lith. apuszis.] (Bot.)
   One of several species of poplar bearing this name,
   especially the {Populus tremula}, so called from the
   trembling of its leaves, which move with the slightest
   impulse of the air.

Source : WordNet®

Populus tremula
     n : Old World aspen with a broad much-branched crown;
         northwestern Europe and Siberia to North Africa [syn: {quaking
         aspen}, {European quaking aspen}]