From cohesive +‎ -ity.

cohesivity (uncountable)

1. cohesiveness
   • 2015 February 23, G. Patrick Maxwell et al., “Ten-Year Results From the Natrelle 410 Anatomical Form-Stable Silicone Breast Implant Core Study”, in Aesthetic Surgery Journal‎^[1], volume 35, →DOI:

     A greater degree of crosslinking of the silicone polymers during the manufacturing process determines the firmness, or cohesivity, of the silicone gel.