The definition of the ical tag is:
Apple's calendar application, bundled with every Mac OS X install
Whereas the calendar tag is:
A calendar is visual representation of time, often charted by months, weeks, or days. Often, a calendar app will help one organize events on a calendar, and sometimes provide alerts or reminders as well.
However, with the advent of Mountain Lion (and, indeed, even with iOS for a while now),
Calendar is the name of the
iCal application. I imagine Apple's intent is to phase out the use of the name
iCal as it homogenizes the platforms.
The same is true of the address-book tag:
Apple's built-in contacts manager software for Mac OS X.
vs. the contacts tag:
Information stored on a computer about various methods to contact a person, such as e-mail, various chat networks, telephone, physical address, etc.
Contacts is now the name of the
Address Book application.
I imagine that simply merging the tags isn't a workable solution, as questions previously tagged with calendar or contacts aren't guaranteed to actually be referring to the formerly-named
Address Book applications.
Going forward, however, new users on Mountain Lion and iOS devices will probably be less and less familiar with the terms
Address Book. How should the community address this with the tags?