Definition:Friction/Coefficient/Static

Definition
Let $B$ be a body at rest on a plane surface $S$ on which friction acts.

Let $N$ be the normal reaction of $S$ on $B$.

Let a force be applied to $B$ parallel to $S$ Let $F$ be the magnitude of that force in the limiting case when $B$ is just about to move.

Then the coefficient of static friction is defined and denoted:
 * $\mu_s = \dfrac F N$

Also see

 * Definition:Coefficient of Kinetic Friction