Functor is a notion which DiSciullo & Williams adopt from Categorial Grammar to express relationships between entities.
if one wishes to express the idea that y is the sister of x, the property 'sister of' can be formalized by using a functor, say F, and write 'y=F(x)'. See Function composition.
- Di Sciullo, A. M. and E. Williams 1987. On the Definition of Word, MIT-press, Cambridge, Mass.