International
Tables for Crystallography Volume C Mathematical, physical and chemical tables Edited by E. Prince © International Union of Crystallography 2006 
International Tables for Crystallography (2006). Vol. C, ch. 6.2, p. 596

There has been some argument over the meaning to be attached to the term Lorentz factor, probably because Lorentz did not publish his results in the ordinary way; they appear in a note added in proof to a paper on temperature effects by Debye (1914). Ordinarily, Lorentz factor is used for the trigonometric part of the angularvelocity factor, or its equivalent, if the sample is stationary. (See below).
References
Debye, P. (1914). Interferenz von Röntgenstrahlen und Wärmebewegung. Ann. Phys. (Leipzig), 43, 49–95.