what is the meaning of "reduced" in term "reduced frequency"
 

:confused:
Something reduced the frequency or compared with something it was reduced?
I just want to know how this term gets its name.

Usually, it means normalized with respect to something.

Its looks like a good explanation,thanks for your reply!

Based on the definition of reduced frequency, if the object moving in the still fluid, then we can consider the frequency as the real physical frequency.

However, the fluid usually has some velocity, imagine that if the freestream velocity is huge(approaching infinite), then the moving frequency is suspended and inferior to the freestream velocity. That is, the freestream velocity looks 'bad' or 'negative' to the moving, I think that is the reason we call the frequency as reduced frequency.