Agility refers to the reactive abilities in unpredictable environments.
You often hear agility mentioned along with speed and quickness, known as SAQ training. Sometime SAQ may even include some jump training and shock method training so lines become blurred and proper application of each quality is missed. It is true that both speed and quickness are components involved with agility but you need to look at agility training as a specific skill that requires an progressive, integrative approach.
True assessment of agility maybe difficult due to the nature of the skill itself. There maybe some tests that can be applied to asses the readiness of the individual to apply specific agility training.
Agility is the ability to start (or accelerate), stop (or decelerate and stabilize) and change direction quickly and to apply these in an unpredictable environment.