Chapter 6 Deploying Wireless Networks for FortiOS 5.0 : Introduction to wireless networking : Security : Captive portal
  
Captive portal
As part of authenticating your users, you might want them to view a web page containing your acceptable use policy or other information. This is called a captive portal. No matter what URL the user initially requested, the portal page is returned. Only after authenticating and agreeing to usage terms can the user access other web resources.
For information about setting up a captive portal, see “Captive Portal security”.