JS error " is not defined" when trying to execute an Ajax call

 This is an issue that has bugged me repeatedly, and I can never seem to find a consistent cause for it. I write up my web service file, write my JS to call it, and it comes back with that "<namespace> is not defined" error. My service file:
 
 
1 using System;
2 using System.Collections;
3 using System.ComponentModel;
4 using System.Data;
5 using System.Linq;
6 using System.Web;
7 using System.Web.Services;
8 using System.Web.Services.Protocols;
9 using System.Xml.Linq;
10
11 nam ...