According to Merriam-Webster:

  mainframe (noun)
  main· frame ˈmān-ˌfrām 

  1: a large, powerful computer that can handle many tasks concurrently and is usually used commercially
  2: (dated): a computer with its cabinet and internal circuits especially when considered separately from any peripherals connected to the computer

The features you list are great to have, but my setup fits the first definition of mainframe as described. If you feel this definition is not specific enough, email Merriam-Webster and don't bother me about it.