Self-stimulatory behavior, also known as stimming and self-stimulation, is the repetition of physical movements, sounds, or repetitive movement of objects common in individuals with developmental disabilities, but most prevalent in autistic people/people with autism spectrum disorders. It is considered a way in which autistic people and others calm and stimulate themselves. Therapists view this behavior as a protective response to being overly sensitive to stimuli, with which the individual blocks less predictable environmental stimuli.Sensory processing disorder is also given as a reason by some therapists for the condition. Another theory is that stimming is a way to relieve anxiety, and other emotions.Stimming exists on a Spectrum. Autism, Aspergers and SPD (Sensory Processing Disorder) become diagnosable when symptoms impair one or more domains of function.
Common stimming behaviors (sometimes called stims) include hand flapping, rocking, excessive or hard blinking, pacing, head banging, repeating noises or words, snapping fingers, and spinning objects. Stimming is almost always an aspect of autism, but it is also regarded as part of some non-autistic individuals' behavioral patterns. The biggest difference between autistic and non-autistic stimming is the type of stim and the quantity of stimming. when the need to stim or the amount of stimming interferes with normal behavior, it becomes diagnosable as autism, Aspergers or SPD (not yet recognized in DSM).
In the Diagnostic and Statistical Manual of Mental Disorders, published by the American Psychiatric Association, this type of behavior is listed as one of the symptoms of autism or "stereotyped and repetitive motor mannerisms". There are numerous ways to reduce or eliminate stereotypic behaviors. Some of them include providing an individual with alternative forms of stimulation; drugs have been used to reduce stimming (however, it is not clear whether the drugs are actually beneficial or restrict the individual from finding relief). The majority of the autistic community opposes attempts to reduce or eliminate stimming as stimming is an important tool for self-regulation.Many contend that attempts at cessation of stimming is potentially abusive, This is one of the major reasons that the autistic community tends to oppose ABA therapy (among a plethora of other reasons including the use of shock treatments and the very basis of ABA--compliance training). 
Stimming can, in some cases, be a self-injurious behavior. Common forms of these behaviors include head-banging, hand-biting, and excessive self-rubbing and scratching. These behaviors, when viewed through the lense of SPD and autism, can be seen as an adaptive response to over and underwhelming interpretation of sensory stimuli.