This term is used very often in the tree care industry, so what exactly is it?
The drip line is the circumference around the outermost branches of the tree. So wherever the farthest reaching branches go all around the tree is your drip line.
So what's the significance?
The drip line contains the vast majority of water and nutrient absorbing roots that are close to the surface. So to water and fertilize efficiently, it should be done within the drip line.
It should also be noted that any soil disturbance done within the drip line of a tree, no matter the size, can have direct negative affects.