iChat can join the AIM (AOL Instant Messenger) service with either names registered with AOL or AIM and also Apple Issued IDs such as those ending in @mac.com and @me.com or in the case of Lion (OS X 10.7.x) @icloud.com names as well.
A few years ago now Apple ran the .Mac service for storing some things on line.
You could pay for an account to do this and get the email and some software downloads and have your website with Apple.
You could also get Trial accounts which worked as emails fro 60 days then reverted to AIM valid names only.
Then Apple changed the services to MobileMe.
This offered mostly the same things but the Trial came to an end after the 60 days and you could not use the name anywhere else.
People moving from @mac.com to MobileMe and the @me.com emails could link them. (emails addressed to either would get to you.)
For use in iChat though the names are seen as two different Screen Names by the AIM service.
Since then Apple have changed to the iCloud service which is free.
People with @me.com names could "registered" those IDs as iCloud Names.
You could also "Convert" any other Apple ID to iCloud as well.
This would give your an iCloud issued @me.com.
Later Apple deciced to issue @icloud.come names.
In some cases this may mean you have @mac.com, @me.com and @icloud.com endings to your Apple ID and they are valid with AIM as three separate AIM Screen Names
IF AOL is your ISP and you have a username from them you may need to enable Messaging on your Account settings and then just the username part will be the Screen Name.
@me.com and @icloud.com name issued by Apple only work in iChat 6 (Lion) and Messages in Mountain Lion. This is due to a second login the App does with an Apple Server at the same time as logging in to AIM to confirm AIM can "See" the password.
Also Apple allows much longer passwords than AIM which limits them to 16 characters or less.
You may have to shorten your Apple Password to work.
My Apple ID is based on an external email.
Although now linked to an iCloud issued @me.com and @icloud.com names as well it in itself is not a valid AIM Screen Name.
Apple IDs ending in @mac.com, @me.com and @icloud.com are valid as AIM Screen Names.
With the latter two these only work in iChat 6 or above.
In all cases the password has to be 16 characters or less.
An AOL as ISP username will also be valid.
You may need to Enable Messaging and only use the Username part of your ID.
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