Avoidance conditioning, as part of classical conditioning, refers to a process in which a subject is taught to avoid "punishment" as part of a two-part process; a neutral stimulus (such as a bell or a light) that precedes the unpleasant "punishment" (such as an electric shock). The subject quickly learns to avoid the punishment by responding to the neutral stimulus.
An example of this would be conditioning a rat to move from one part of a cage to another after hearing a bell ring. The bell ring (the neutral stimulus) is presented to the rat immediately before it feels an electrical shock. The rat can avoid the shock if it moves to another part of the cage. Through avoidance conditioning the rat will eventually learn to move to the non-shock area of the cage as soon as it hears the bell.