What is the industry standard or best practice for defining
availability requirments of a computer system?  For example the
highest level of availability is called _____ and involves clustering
to ensure 24/7 uptime, the second  highest is.... and involves.....

I don't know of any "standards" here. MSF (Microsoft Solution
Framework) is a good model that standardizes a lot of the requirements
analysis. http://www.microsoft.com/msf

I should mention; however, the term "Enterprise" is a standard
architecture term and implies that 24/7 uptime is required. However,
the definition is becoming broader as more developers are creating
larger applications that fall into the pseudo-enterprise category.