$ is not defined error

[eluser]JasmineFlower[/eluser]
Hi,

I am using codeigniter1.7.3. In my project i am using jquery ajax for form login validation, but in firefox error console displays the error - $ is not defined.
Pls anyone know about this tell me how to solve.My ajax code as below

Code:

$.ajax({
  type: 'POST',
  dataType: 'json',
  url: base_url + 'home/loginauth/' + Math.random(),
  data: 'email=' + em+'&pword;=' +pw,
  success: function(response)
  {    
   var urlcontent = response.result;      
   alert(urlcontent);
   if(urlcontent == 0)
   {
    document.getElementById('error_msg')[removed]="Invalid username and password";
    return false;
   }
   else
   {  
    
    document.location= base_url + 'login/index/'+ urlcontent;
    return true;
    
   }
  
  
  }
  
  });