Plug compatible
From Computer History Wiki
Plug compatible peripherals (the term was not usually used of other items, like main memory) were devices made by third-party manufacturers which could be plugged into systems made by another manufacturer, and operate in every way as if they were the equivalent units from that manufacturer (hence the name). They were usually cheaper than the units from the original manufacturer, which was why users were willing to take the risk of using an 'off-brand' alternative.
Disks and magnetic tape drives were the most common peripherals to be found in the plug compatible market. That market got started with units for IBM mainframes, but rapidly expanded to other manufacturers, such as DEC.
