id·i·o·type(d–tp)
n.
A set of one or more antigenic determinants specific to the variable region of an immunoglobulin molecule.

The American Heritage® Dictionary of the English Language, Fourth Edition copyright ©2000 by Houghton Mifflin Company. Updated in 2003. Published by Houghton Mifflin Company. All rights reserved.

http://pathmicro.med.sc.edu/book/immunol-sta.htm

I don’t know how good the above online text is, but the price is right (free). You can look up the basics of immunology there.

This what I can remember, I hope it helps. Hmm you know that antibody bind to antigens right. But did you know you can make antibodies attack antibodies too. idiotypes are antibodies’s varible regions (their own antigen recognition site), which are being attacked by other antibodies. So basically antibodies’ variable region are themselves being used as antigen sites. Maybe I can rephrase it a little.

http://en.wikipedia.org/wiki/Antibodies

yeah i wrote too much. Just go with the definition of it being different variable regions.

Guys? Do textbooks exist for you?

I did not look in the textbook for this one because I constantly use antibodies for blotting experiments, and idiotype was just something that came to me.