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Atomic displacement parameter (also called displacement factor, or called B-factor or often incorrectly called temperature factor), β, describes the uncertainty in the position of an atom within a crystal. This uncertainty may originate from thermal motion of the atom, resulting in variations in the position of the atom between different copies of the unit cell, or may originate from defects in the crystal. The thermal motion and position uncertainty may be isotropic or anisotropic. Anisotropic atomic displacement parameters are denoted by 6 parameters:
        i) 3 variances (x, y and z).
        ii) 3 covariances of the possible pairs of coordinates x, y and z.

Mean-square atomic displacement <u²> values can be experimentally obtained from various measurements, such as thermal expansion, thermal conductivity, extended x-ray absorption fine structure (EXAFS) and the widths of the x-ray or neutron diffraction peaks.