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