# Dimensionless wall distance (y plus)

$y^+ \equiv \frac{u_* \, y}{\nu}$
Where $u_*$ is the friction velocity at the nearest wall, $y$ is the distance to the nearest wall and $\nu$ is the local kinematic viscosity of the fluid.
$y^+$ is often refered to simply as y plus and is commonly used in boundary layer theory and in defining the law of the wall.