Updated April 02, 2015.

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

If you know anything at all about autism, you know that it's a disorder often referred to as "the autism spectrum."

If you've hung around groups of people interested in autism, you've even heard the expression shortened to "The Spectrum." As in "he's on the spectrum," or "it's a great program for kids on the spectrum."

What are they talking about?

In general conversation, the word "spectrum" means the same thing as "a wide range of."  As in "the paints come in a spectrum of colors." Spectra (plural of spectrum) are usually represented by a rainbow shape, with one extreme at one end of the spectrum and the other extreme at the other end.

In the world of physics, the word spectrum almost always refers to the light spectrum -- the colors of the rainbow plus the invisible rays that include ultrared, ultraviolet, X-rays, gamma rays, and so forth. The shortest light frequencies are at one end of the spectrum, while the longest frequencies are at the other end.

In the world of medicine, the word "spectrum" also refers to diversity within a diagnostic category.

Diversity within a diagnostic category is by no means unusual. Even within a category such as "headache," for example, which obviously refers to "pain in the head," there are a wide range of symptoms. There are, for example, people with mild headaches and people with severe headaches.

So why not just say "mild" or "severe," and leave it at that?

The term "spectrum" is used when "mild" and "severe" don't cover the entire set of possibilities. Typically, it's used to allow for the possibility of "mildish" or "moderate" or "rather but not very severe" symptoms. It's also used to suggest that there are many different possible combinations of symptoms.

To return to the example of headaches -- not only are headaches mild, moderate, and severe, but they are also different in quality and cause. One person may have a stabbing headache while another has a dull ache. One person's headache may be set off by chemical exposure while another person's headache is caused by stress.

Many disorders, particularly mental and developmental disorders, include a wide array of people with many different types and levels of symptoms. There is an Obsessive Compulsive Disorder Spectrum, a Schizophrenia Spectrum, and a Bipolar Disorder Spectrum. People with all of these disorders have some symptoms in common but -- as with headache -- the severity, quality, and cause may vary.

The autism spectrum, because it has become a relatively common diagnosis, has the special distinction of being THE Spectrum to many people. The Spectrum is huge, encompassing people who are unusually intelligent and people who are cognitively challenged; people who are gentle and people who are aggressive; people who enjoy roller coasters and people who can't bear motion. As you've no doubt heard already (many times!) - when you know one person with autism, you know one person with autism.

While the idea of an autism spectrum is useful in many ways, it can also be very confusing. For example, it begs such questions as --

• How can all the people on this spectrum really be autistic?
• How can you discuss or represent a group of people who are so diverse?
• Is it really possible to do research on "people with autism" if they are so different from one another?
• How can programs, supports, or opportunities be designed for "people with autism" when they have so little in common with one another?

As long as the autism spectrum is defined as a single diagnostic category, these questions will not go away. Is The Spectrum really too broad? The jury is still out.