When the fluidizing agent is given, the minimum fluidization velocity (Umf) of particles is generally reckoned to be the sole function of their physical properties. However, Umf determined as per its original definition was found to depend also on how the particles were packed before the experimental measurement, causing the well-known poor reproducibility and difficulty in comparing the Umf reported by different scholars. The original definition of Umf neglected the polymorphism of packed particles and did not clearly indicate which of the many states the particles should be packed into before the experimental measurement. To solve this problem, this work proposed a precise definition for Umf by specifying the initial packing of particles: Umf characterizes the state transition of particles from "free" packed to fluidized. Such a definition could endow Umf with a clear physical meaning and a definite value. However, as for mixed particles, no precise and realizable definition for apparent Umf could be proposed yet. The difficulty lies in specifying a reproducible packed state as the starting point for the experimental measurement, which, first and foremost, should have good mixing. Under such circumstances, a compromise method was proposed to improve the reproducibility of measuring Umf as per its preliminary definition. [ABSTRACT FROM AUTHOR]