Unit of the resistance (3)
I believe the answer is:
ohm
I believe this is a double definition.
'unit' is the first definition.
(ohm is a kind of unit)
'the resistance' is the second definition.
(thesaurus)
'of' acts as a link.
(Other definitions for ohm that I've seen before include "Unit of electrical resistance (3)" , "A little resistance" , "physical unit" , "lawmaker" , "Physicist" .)