The lower limb Mechanical Axis is defined by a line running through the centre of the hip joint to the centre of the ankle joint. This line should pass essentially through the centre of the knee joint.
The tangential distance of this line from the centre of the knee joint is referred to as the Mechanical Axis Deviation.
In a Varus knee, the axis passes through the knee medial compartment, while in a Valgus knee it crosses the knee lateral side.
Another key concept is the one of Anatomical Axis. The Anatomical Axis of a bone is represented by a line drawn down the centre of the medullary cavity of the bone.
In the femur, the mechanical and anatomical axes deviate whereas in the tibia the mechanical and anatomical axes are essentially represented by the same line.
The angle between the Anatomical Axis of the femur and the lower limb Mechanical Axis is normally 6° of Valgus.
source: this website