An axial rift is a linear, narrow trough or valley formed at a divergent plate boundary where two tectonic plates are pulling apart. These rifts are typically found along mid-ocean ridges and are the sites of active volcanism and crustal formation. The study of axial rifts is important for understanding the processes of seafloor spreading and the formation of new oceanic crust.
Reference: Macdonald, K. C. (1982). “Mid-Ocean Ridges: Fine Scale Tectonic, Volcanic and Hydrothermal Processes within the Plate Boundary Zone.” Annual Review of Earth and Planetary Sciences, 10, 155-190.