The loss tangent or tan d is defined as the ratio of the imaginary part of the dielectric constant to the real part. D denotes dissipation factor and Q is quality factor. The loss tangent tan d is called tan delta, tangent loss or dissipation factor. Sometimes the term “quality factor or Q-factor” is used with respect to an electronic microwave material, which is the reciprocal of the loss tangent. For very low loss materials, since tan d ≈ d, the loss tangent can be expressed in angle units, milliradians or microradians.