Official comment
Did you ask this on Reddit? If it wasn't you, read the answer Profbx (one of Pioneer DJ's very own product planners) provided:
What led Pioneer to chose the term "Master Tempo" instead of "Key Lock" like the rest of the universe? The term "Master Tempo" makes my brain hurt. I've tried hard to come up with some contrived logic to relate the term "Master Tempo" to the function it supposedly describes and I can't. Everybody knows what Master Tempo does, but nobody seems to know why it's called that. The only logical explanation seems to be someone at Pioneer got really high when they came up with this name and, in that state of mind, it made perfect sense.
Did you ask this on Reddit? If it wasn't you, read the answer Profbx (one of Pioneer DJ's very own product planners) provided:
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The CDJ-500, the first CDJ ever, released in 1994, also has a button labelled Master Tempo. I just downloaded the user manual and it says:
MASTER TEMPO button (M●T)
Each time this button is pressed, the MASTER TEMPO function (*3) is set to ON or OFF. When the MASTER TEMPO function is set to ON, the MASTER TEMPO indicator in the display window lights.(*3) MASTER TEMPO function: Even though playback tempo is changed with the TEMPO control knob, the playback pitch is kept fixed.
Note: I think by tempo control knob they meant slider/fader.
After reading this definition of Master Tempo, the term "Master Tempo' still doesn't make sense.