I recently applied for a apartment and was rejected. The reason they gave me was stated in one word: criminal. I have a felony conviction almost 10 years old. Is this a form of discrimination?
Yes, it is a form of discrimination. But unfortunately, in all likelihood you have no legal protection against it.
Only certain kinds of discrimination, such as that based on race, ethnicity, religion, family status and disability are recognized as illegal. Many states and some localities have added others -- such as gender and sexual orientation -- but convicted felons are not now on any state's list. (Under federal law, however, it is illegal for a landlord to discriminate against a tenant who has a felony conviction for drug use -- but a landlord can discriminate against someone with a felony record for drug sale or manufacture.) It is very unfair, in many cases, to deny housing to someone based on a very old conviction, but there is no law against it.
In the future, anticipate that this information will come to the attention of prospective landlords -- and employers, too. Deal with it before the landlord or employer sees it on your credit report or comes across it during some other check. Bring up the fact of the conviction yourself, during the application process, and do your best to convince the landlord that you have moved well beyond those days and will be a thoroughly law-abiding and stable tenant. If the conviction is old and your record is spotless since you got it, it will be harder for the landlord to reject you.