Question

what is water divide is Himalaya is a water divide give reason

Answer 1

A perfect water divide would mean that the tops of the mountains are all the same height and are all in perfect alignment and in addition to this 'geometric' configuration, the tops of all of the Himalayan mountains would have to contain the same exact rock type (preferably a fully non-porous rock so that precipitation in the form of snow or rain does not penetrate the ground but rather flows across the rock surface) so that all rain or snow would flow down one of only two possible directions down this hypothetical perfect water divide. 

In summary, nothing in nature is ever perfect; rain or snow falling on the Himalaya's does not ONLY flow in two opposite directions -- therefore the Himalayas are not a perfect water divide

Answer 2

In topography, a drainage divide, water divide, divide, ridgeline, watershed, water parting or height of land is elevated terrain that separates neighboring drainage basins.