The Collaborative International Dictionary of English v.0.48:

Jalap \Jal"ap\, n. [F., fr. Sp. jalapa; -- so called from
   Jalapa, a town in Mexico, whence it was first obtained.]
   (Med.)
   The tubers of the Mexican plant Ipom[oe]a purga (or
   Exogonium purga) of the family Convolvulaceae, a climber
   much like the morning-glory. The abstract, extract, and
   powder, prepared from the tubers, are well known purgative
   (cathartic) medicines, and are also called jalap. Other
   species of Ipom[oe]a yield several inferior kinds of jalap,
   as the Ipom[oe]a Orizabensis, and Ipom[oe]a tuberosa.
   [1913 Webster]

   False jalap, the root of Mirabilis Jalapa, four-o'clock,
      or marvel of Peru.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

marvel of Peru \marvel of Peru\, marvel-of-Peru \marvel-of-Peru\
   n. (Bot.)
   A common garden plant (Mirabilis jalapa or Mirabilis
   uniflora) of North America having fragrant red or purple or
   yellow or white flowers that open in late afternoon; called
   also four-o'clock. See four-o'clock.

   Syn: common four-o'clock, marvel of Peru, Mirabilis jalapa,
        Mirabilis uniflora.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Mirabilis jalapa
    n 1: common garden plant of North America having fragrant red or
         purple or yellow or white flowers that open in late
         afternoon [syn: common four-o'clock, marvel-of-Peru,
         Mirabilis jalapa, Mirabilis uniflora]