XHTML 1.0 Validation error?

I received this explanation for why my page did NOT validate as XHTML
Strict:
 
Below are the results of attempting to parse this document with an SGML
parser. 
1.	Line 69 <http://validator.w3.org/check#line-69> , column 50:
there is no attribute "target" (explain...
<http://validator.w3.org/docs/errors.html#attr-undef> ). 
        <li><a href="http://www.thetanu.org" target="_blank">Theta
Nu.org</a></li>
 
I assume this to be an error, as the rest of my similar links did not
incur an error.
 
--
 
Ryan Cannon
Freelance Web Design
Homepage <http://www.ryancannon.com> 
(734)771-2395