also: しょうう · こあめ
Refers to rain that is not heavy, typically with small drops and low intensity. The reading こさめ is the everyday term; しょうう is literary/formal, and こあめ is rare.
A light rain has been falling since morning.
It's pleasant to take a walk in a drizzle.
It has been drizzling since morning.
I walked in the drizzle without even opening an umbrella.
By the afternoon the rain will probably ease into a drizzle.
The drizzle finally stopped.