what classifies a computer as a mainframe?

1. A mainframe has 1 to 16 CPU's (modern machines more)
2. Memory ranges from 128 Mb over 8 Gigabyte on line RAM
3. Its processing power ranges from 80 over 550 Mips
4. It has often different cabinets for
   Storage
   I/O
   RAM
5. Separate processes (program) for
   task management
   program management
   job management
   serialization
   catalogs
   inter address space
   communication

Historically, a mainframe is associated with centralized computing opposite from distributed computing. Meaning all computing takes (physically) place on the mainframe itself.