What does 127 stand for in BY 127 diode?

Answer 1

BIPOLAR

A2

It does not stand for anything. It is a maker's designation mark, a throwback from the thermionic tube (valve) days. The letters in front of the number used to indicate the heater voltage and the type of internals (triode, pentode etc)

B used to indicate that it was a semiconductor, not a tube.

There are different designations around the world, for the same equivalent device, American, European and Japanese.