This is taken from Spinoza's Ethics:
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 ?