All that said, however, there are no such diagnoses as "mild autism" or "high functioning autism."
So what does a practitioner, teacher or parent mean when they say their child (or your child) has "mild" autism?
Since there is no official definition of the term "mild autism," every person using it has a slightly different idea of what it means. Sometimes the term is used instead of Asperger syndrome because it's more generally understood (Asperger syndrome is still a relatively obscure diagnosis). Sometimes the term is used when an individual is clearly autistic, but also has significant spoken language and other skills. The term may also be used euphemistically to describe a child whose challenges are by no means mild, but who has just a few spoken words.
To make matters more difficult, a person with "mild autism" may have advanced communication skills and academic abilities, but have very delayed social skills, severe sensory issues, and/or extreme difficulties with organizational skills. As a result, the individual with "mild" autism may find public school or work settings more challenging than an individual with greater language challenges but fewer sensory or social problems.
As an example, imagine a very academically bright, linguistically advanced individual who is suspicious of his peers' motives and can't abide the sound of a vacuum cleaner or the light of a fluorescent bulb. Compare such a person to an individual who can carry on a basic conversation, has few issues with sound or light, and is generally friendly in the classroom or workplace. Which individual has "milder" symptoms? The answer, of course, is that it depends upon the setting and the situation.
Bottom line, the term "mild autism" is not especially useful, though it is fairly common. To really make sense of its significance, you'll need to ask direct, specific questions about verbal, social, sensory and behavioral challenges.
Attwood, Tony. The Complete Guide to Asperger Syndrome. Jessica Kingsley Press:2007.
Koyama T, Tachimori H, Osada H, Takeda T, Kurita H. "Cognitive and symptom profiles in Asperger's syndrome and high-functioning autism." Psychiatry Clin Neurosci. 2007 Feb;61(1):99-104.
Seung HK. "Linguistic characteristics of individuals with high functioning autism and Asperger syndrome." Clin Linguist Phon. 2007 Apr;21(4):247-59.