Free relative is relative clause that occurs without an antecedent.
the subject in (i), who finishes first, is a free relative.
(i) [who finishes first] wins the prize
Usually one assumes that free relatives are NPs with an empty head noun: [NP 0 [S who finishes first]].
- Smits, R.J.C. 1989. The relative and cleft constructions of the Germanic and Romance languages, doct. diss. KUB, Tilburg, Dordrecht: Foris, 1988.