wbmw, By the way, a mainframe most certainly is considered a server.
Yes, although that is a matter of branding as much as anything else. Mainframe was the accepted term for a computer which ran applications and interacted with the user through some sort of terminal device (cards, tapes, APL terminal, etc.).
Networks evolved around minicomputers first, and microcomputers soon afterwards, which distributed processing. A server thus was invented as a common storage receptacle and for running mundane jobs like printing. Soon email was invented as the first real server application.
Now, IBM was late to the networking party and was selling mainframes hookup up to dumb terminals way after everyone else moved on. Then when IBM was ready to join the party, they tried to hijack it with Token Ring and SNA. A very few years later they realized that Ethernet was going to predominate and they'd better start learning the lingo of the new order.
Hence IBM mainframes where rechristened 'servers' and served as application servers for PCs connected by Ethernet.
So mainframes are servers because IBM says so.
:)