systematic desensitization refers to behavioral therapy technique to diminish excessive fears, involving gradual exposure to the feared stimulus paired with a positive coping experience, usually relaxation.
Other /More definition:
Systematic desensitization refers to a three-step behavior therapy technique for treating anxiety whereby (1) the child is taught to relax, (2) an anxiety hierarchy is constructed, and (3) the anxiety-provoking stimuli are presented sequentially while the child remains relaxed. Systematic desensitization is a variant of counterconditioning that involves presenting the client with progressively more anxiety-provoking images while in a relaxed state.