"As allowed".A quantum fluctuation is the temporary appearance of energetic particles out of empty space, as allowed by the uncertainty principle.
Because they observe it, they can say "this is going on!", then they put this label on the phenomenon as being "allowed", even if it's in fact pure magic, and nobody can explain why.
Most of today's science it the same. Students are taught that certain things are "known" and "established", and they should not research into them else they will become heretics, even if all that is known is what happens and not why.
For example (from wikipedia):
Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field.
Looks kind of circular to me.An electromagnetic field (also EM field) is a classical (i.e. non-quantum) field produced by moving electric charges
Clearly they see and can measure what's going on, but as we see from the definitions they have no idea why.