Question

define water retention in soil

Answer 1

Soil water retention means capacity of the soil to retain the water in it.
It fully depends on the size of the soil particles. If the soil particles are small they will pack tightly hence leaving no or very less space between them hence not allowing the water to pass through them.
If the size of the particles is large they will not pack tightly and leave more space in between them (pores) that allow the flow of water through them and hence have low water retention capacity.