Mainframe, MVS and zOS Discussion

Hi,

What exactly is a Mainframe? I read about it online and some says - 1. It is a very big computer. 2. It's meant for business usage. 3. It's like a super computer.

Question is if it's something else or other, why was it invented and what keeps it still relevant?

By the way I found this site too while searching about my question and thought this site can give a better answer.

Mainframe is a big computer used by banks and the big companies in the world.