(Science: physics) A charged particle travelling into an increasing magnetic field will (if the field becomes strong enough) reverse direction and be reflected back. This is a direct result of the adiabatic invariance of the magnetic moment. Plasmas can be confined by devices which utilise this effect.
The effect also occurs in some toroidal plasmas, since the toroidal magnetic field is stronger on the inboard side than on the outboard side, in this case it gives rise to so-called neoclassical effects. The strength of the mirror is determined by the mirror ratio.