Boost mixing of E and B
The phenomenon that under a Lorentz boost, electric and magnetic field components rotate into each other. A pure-E field in one frame becomes E and B in any boosted frame; a pure-B field becomes E and B; the two are not separate physical entities.
Definition
Boost mixing of E and B is the phenomenon that the components of the electric and magnetic fields perpendicular to a Lorentz boost direction rotate into each other under that boost, so a configuration that appears purely electric in one inertial frame appears as a mixture of electric and magnetic fields in any frame moving past, and vice versa. The transformation is closed-form (see "Lorentz transformation of fields"), but the conceptual content is that E and B are not two independent physical objects — they are projections of a single rank-2 antisymmetric tensor F^{μν} onto the time and space directions of a particular observer's reference frame.
The most striking manifestation is in §11.4 magnetism-as-relativistic-electrostatics: two parallel current-carrying wires in the lab frame produce a magnetic attraction (no electric force, because both wires are net-neutral). Boost into the rest frame of the drift electrons in one wire and the lattice ions of the other wire are now moving — their length contracts by γ, the lattice line density increases, and a net electric field appears between the wires. The "magnetic" attraction in the lab frame becomes a "Coulomb" attraction in the boosted frame, with the same magnitude. Same force, different name. The phenomenon also explains why a moving observer near a stationary charge sees a magnetic field (the test charge's worldline pierces the boosted-frame's slicing of spacetime, picking up a current density that sources a B-field) and why the apparent type of field is not a frame-independent classification — only the two scalar invariants E·B and |E|² − c²|B|² are.