In contrast to bound, the term free refers to the lack of binding acting on a particular clause or noun phrase. In other words, free is the opposite of bound and therefore does not allow for a given noun phrase to be coindexed by a c-commanding phrase. (Carnie, 2006, p. 155)
Carnie, A. (2006). Syntax: A generative introduction. John Wiley & Sons.
Lai, I. S. (1998). The grammar and acquisition of Secwepemctsín independent pronouns. [Master’s thesis, The University of British Columbia].