It consists of an internal margin added to the CTV to compensate for internal physiologic movement and variations in size, shape, and position of the CTV [1]. It's an expansion of CTV for internal (e.g. breathing) movement [2].
Bibliographic references:
[1] Reiff J. E. (2013). C. In: L. Brady and T. Yaeger, ed., Encyclopedia of Radiation Oncology, 1st ed. Springer-Verlag Berlin Heidelberg, pp.343-399.
[2] Balter J. Target definition (margin selection) for radiotherapy (IMRT). aapmorg. 2017. Available at: https://www.aapm.org/meetings/03SS/Presentations/Balter.pdf. Accessed September 11, 2017.