Motif(r) is the industry standard graphical user interface, (as defined by
the IEEE 1295 specification), used on more than 200 hardware and software
platforms.  It provides application developers, end users, and system
vendors with the industry's most widely used environment for standardizing
application presentation on a wide range of platforms.  Motif is the
leading user interface toolkit for the UNIX(r) system.
 
NOTE: Some ports with GNU configure do not know that Motif 2.1 requires -lXp.
In that case, you need to edit Makefile after configure, or, hack
configure(.in) before configure.

WWW: http://www.opengroup.org/openmotif/