Autism was first described as a distinct medical condition by the Austrian- American psychiatrist Leo Kanner in 1943. In his seminal paper, Kanner identified 11 children who exhibited behaviors such as extreme social withdrawal, delayed or unusual speech patterns, repetitive behaviors, and a strong need for sameness, which he termed "early infantile autism".

Prior to Kanner's work, in 1911, Swiss psychiatrist Eugen Bleuler had introduced the term "autism" but only as a symptom of schizophrenia, not as a separate condition. The concept of autism as an independent condition was thus established in the 1940s with Kanner's diagnosis.

Autism became officially recognized as a diagnosis in the Diagnostic and Statistical Manual of Mental Disorders (DSM) starting with DSM-III in 1980, which marked its identification as a distinct developmental disorder.

In summary, autism was first diagnosed as a separate condition in 1943 by Leo Kanner, with its official diagnostic classification emerging in 1980.