ACS is short for Windows Azure Access Control Service
The Windows Azure Access Control Service provides federated authentication using Google, Yahoo, Live Id, Facebook and other WS-Federation partners such as ADFS.
ACS has been deprecated in favor of Azure AD. The functionality of ACS is being merged into AAD.
http://blogs.technet.com/b/ad/archive/2015/02/12/the-future-of-azure-acs-is-azure-active-directory.aspx