Master / Slave is a device status on a device controller. It is  
usually with hard drives.
On many older shared controllers you could plug in two devices (hard  
drive, optical drive, floppy, etc...)
One connector was the master, and it got priority communication with  
the system (and generally more bandwidth) the other device was a  
slave and got less.
In the 90's it became controversial although the term was around for  
a LONG time and only implied the relationship between the devices.