1. （尤指战时）使落叶，去掉…的叶子，毁坏(某一地区)的绿色植物
To defoliate an area or the plants in it means to cause the leaves on the plants to fall off or be destroyed. This is done especially in war to remove protection from an enemy.

1. strip the leaves or branches from

e.g. defoliate the trees with pesticides