The autism spectrum or autistic spectrum describes a range of conditions classified as pervasive developmental disorders in the Diagnostic and Statistical Manual of Mental Disorders (DSM). Pervasive developmental disorders include autism, Asperger syndrome, pervasive developmental disorder not otherwise specified (PDD-NOS), childhood disintegrative disorder, and Rett syndrome, although usually only the first three conditions are considered part of the autism spectrum. These disorders are typically characterized by social deficits, communication difficulties, stereotyped or repetitive behaviors and interests, and in some cases, cognitive delays. Although these diagnoses share some common features, individuals with these disorders are thought to be "on the spectrum" because of differences in severity across these domains.

Autism is characterized by delays or abnormal functioning before the age of three years in one or more of the following domains: (1) social interaction; (2) communication; and (3) restricted, repetitive, and stereotyped patterns of behavior, interests, and activities. Social impairments are marked by poor use of nonverbal communication, difficulty in peer relations, lack of social-emotional reciprocity, and lack of shared enjoyment. Communication deficits may include failure to develop speech, use of stereotyped or delayed echolalia, and difficulties maintaining conversations. Social and communication impairments may also cause a lack of symbolic or imaginative play. Restricted and repetitive behaviors may include unusual preoccupations with narrow interests, inflexibility to nonfunctional routines, stereotyped and repetitive mannerisms, and preoccupations with parts of objections.

A spectrum as it applies to mental disorder is a range of linked conditions, sometimes also extending to include singular symptoms and traits. The different elements of a spectrum either have a similar appearance or are thought to be caused by the same underlying mechanism. In either case, a spectrum approach is taken because there appears to be "not a unitary disorder but rather a syndrome composed of subgroups". The spectrum may represent a range of severity, comprising relatively "severe" mental disorders through to relatively "mild and nonclinical deficits".

In some cases, a spectrum approach joins together conditions that were previously considered separately. A notable example of this trend is the autism spectrum, where conditions on this spectrum may now all be referred to as autism spectrum disorders. In other cases, what was treated as a single disorder comes to be seen (or seen once again) as comprising a range of types, a notable example being the bipolar spectrum. A spectrum approach may also expand the type or the severity of issues which are included, which may lessen the gap with other diagnoses or with what is considered 'normal'. Proponents of this approach argue that it is in line with evidence of gradations in the type or severity of symptoms in the general population, and helps reduce the stigma associated with a diagnosis. Critics, however, argue that it can take attention and resources away from the most serious conditions associated with the most disability, or on the other hand could unduly medicalize problems which are simply challenges people face in life.