The term 'prefix with ballistics or magnetics' can be solved by the crossword solution AERO, which is a four-letter word. AERO is a prefix that is used in combination with either ballistics or magnetics. Ballistics is the science of the motion of projectiles, such as bullets, and magnetics is the study of the magnetic properties of materials. Therefore, AERO is a prefix that is used to describe something related to either ballistics or magnetics.