The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   Note: The aspen poplar is Populus tremula and Populus
         tremuloides; Balsam poplar is Populus balsamifera;
         Lombardy poplar (Populus dilatata) is a tall, spiry
         tree; white poplar is Populus alba.
         [1913 Webster]

   2. The timber of the tulip tree; -- called also white
      poplar. [U.S.]
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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, [aum]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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Populus tremula
    n 1: Old World aspen with a broad much-branched crown;
         northwestern Europe and Siberia to North Africa [syn:
         quaking aspen, European quaking aspen, Populus
         tremula]