ACE is not an acronym, so it's not short for anything. It's just a name Oracle made up.
Originally, you had to be nominated for ACE, but now you can self nominate. For ACE Director you still need to be nominated. There is a group in Oracle that look at all the nominations and decide if people are worth granting the status too.
ACE directors have some extra responsibilities, like having to attend some conferences for free. Some people cannot or will not take the unpaid time off work, so they can't do this.
The ACE program is explained here:http://www.oracle.com/technology/commun ... index.html