Inertial stresses in Reynolds number

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.
• For the dynamic pressure referenced in the Bernoulli equation, it would indeed carry a factor of $\frac{1}{2}$; however, the typical definition of characteristic dynamic pressure used for the definition of a Reynolds number doesn't. This is because the particular form of Reynolds number the OP describes is $Re = \frac{\rho v^2}{\mu \frac{v}{L}}$, which straightforwardly simplifies to its common form; but the addition of constant flow-independent factors is common and perfectly fine in defining Reynolds numbers as long as you do the proper book-keeping for it. – aghostinthefigures Jul 11 '18 at 3:29