Meaning
Decline, the sun's passing the meridian toward setting; downfall, decay.
Literally: Arabic zawal: the moving away of the sun from its zenith.
How Iqbal uses it
Zawaal is both the sun's descent and the decline of nations and selves; Iqbal warns that whatever rises by borrowed light must set, and calls the self to a glory that knows no zawaal.
See it in the verse