So that's the dictionary.com definition. It's sort of flawed though. By that defiition I'm also retarded in several ways (pretty much any mental disorder can be retarded by that definition). I've never seen any psychiatrist or other medical professional characterize autism as "mental retardation". Nor any other limitation to social or emotional development. It's really more focused on intellectual understanding. Some more clear definitions:

A neurodevelopmental disorder characterized by impaired intellectual and adaptive functioning which is defined by an IQ score below 70 as well as a delay in general daily living skills.

Most folks with Autism don't fit that criteria. Not even within their own age-group. People classified as "mentally retarded" tend to, at all period of their life, be behind people of their age in terms of intellectual understanding. Of course, since things slow down as an adult, they tend to get fairly close or even to the point of a stable adult, and as I said, can perform all standard adult tasks that don't require a greater level of intellect and study (you know, like what most adults do anyways).

The above definition is the more currently-used one. They also don't really call it "retarded" anymore, since stupid kids and trolls co-opted it to be offensive.

Here's another, somewhat dated definition:

less advanced in mental, physical, or social development than is usual for one's age.

This definition refers to general retardation. Physical, social, or mental. Folks with Autism tend to deal with Social retardation. I imagine Cerebral Palsy can be classified as physical retardation, since they don't get progressively worse, but can improve somewhat overtime. Mental retardation generally refers to symptoms common in down syndrome and various other disorders.

Point is, you aren't mentally retarded.