In generative syntax, A-bar movement (or A ′-movement) is movement to an A-bar position (e.g. wh-movement). As is well known, unbounded dependencies are a characteristic property of A ′-movement （Huang et al., 2009). Constructions that have been shown to be derived by A ′-movement are typically labeled as “wh-movement” structures because they are well represented by wh-interrogative constructions in English.
The term was first used in the late 1980s.
The following are characteristics of A ′-movement:
a. A gap exists and has an A ′-antecedent.
b. The antecedent–gap relation can cross multiple clause boundaries – unbounded dependency.
c. The dependency relation is sensitive to locality conditions such as Subjacency and the Condition on Extraction Domain.
Huang, C. T. J., Li, Y. H. A., & Li, Y. (2009). The syntax of Chinese (Vol. 10). Cambridge: Cambridge University Press.