This is taken from Spinoza's Ethics:

notandum est

 Iƒ veram **uniuscujusque** rei definitionem nihil involvere neque exprimere præter rei definitæ naturam.

As I can see, it is Adjective in Genitive and should mean unique, only one of the kind etc. 

I cannot to find anything in my dictionary except  'unus,a,um' : 

Perhaps it comes from unusquisque, if there exists a word like that? Or perhaps certain words, like  'unus'  and  'quis'  here can be casually written together, without breaking any rules ?