What are the units of the coefficient of friction?

Coefficient of friction is a constant ratio of two forces acting on a body at same time.

This ratio is represented by a letter μ. Suppose forces acting on a body due to its surface and due to pressure applied on it be F and L respectively.

Now as μ = F/L and both F and L are measured in term of force, then μ is a dimensionless quantity and do not have any unit.

The coefficient of friction is force between two objects when one object is moving, or if two objects are moving against one another.