Fast Ion Confinement is the physical phenomenon and performance metric in magnetically confined fusion plasmas that characterizes the ability of a stellarator or tokamak magnetic configuration to retain high-energy ions—such as those injected by neutral beam injection—within the core plasma volume over sufficient timescales to contribute effectively to plasma heating and current drive, distinguishing it from general particle or thermal confinement by its specific dependence on the phase-space trajectories and collisional dynamics of non-thermal ion populations.