What is meant by the inertial stress in the definition of Reynolds number? Reynolds number = Inertial stress/ viscous stress
The quick answer to your question is that the "inertial stress" as referenced in the Reynolds number definition is equivalent to a dynamic pressure $\rho v^2$, where $\rho$ and $v$ are some characteristic density and speed of the flow respectively.
Note that this isn't a unique way of defining the Reynolds number; you can do it by taking the ratio of "inertial forces" to viscous forces as in this answer. The key concept is that the Reynolds number compares the inertial effects of a flow to the viscous effects of a flow, and that can be done through any number of equivalent ratios (inertial to viscous length scales, etc.)