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If "Master/Slave" terminology in computing sounds bad now, why not change it to "Dom/Sub"?
(lemmy.dbzer0.com)
A "Showerthought" is a simple term used to describe the thoughts that pop into your head while you're doing everyday things like taking a shower, driving, or just daydreaming. A showerthought should offer a unique perspective on an ordinary part of life.
Primary/secondary?
That doesn't make sense depending on the context. New I2C standard switched to controller/target for example. This conveys that one device is controlling the other devices.
My suggestion doesn't make sense in the classic PATA sense either, since there were potentially several "slave" devices, but they weren't slaves so much as dependent on the "master."
Parent/child(ren)(s)