The symbol mu (or `mu` ) can be used to describe a number of properties in science. It can be used to denote the coefficient of friction of a surface (`mu_s` for coefficient of static friction, etc.) or dynamic (or absolute) viscosity of a fluid. In this case, since only `mu` is given, we can assume it is the dynamic viscosity.
Viscosity is a measure of resistance offered by a fluid to the deformation by stresses. An ideal fluid is inviscid (i.e. has no viscosity). A Newtonian fluid is one in which the rate of deformation (`(du)/(dy)` ) is directly proportional to the shear stress (`tau` ). Dynamic viscosity is the proportionality constant in this equation:
`tau = mu (du)/(dy)`
The units of dynamic viscosity are Poiseuille (SI system) and Poise (CGS system, more commonly used).
Hope this helps.