A balun is an electrical device that converts between a balanced signal (two signals working against each other where ground is irrelevant) and an unbalanced signal (a single signal working against ground or pseudo-ground).
Балун (от — balanced-unbalanced) — жаргонное название симметрирующего трансформатора, преобразующего электрический сигнал из симметричного (balanced) в несимметричный (unbalanced) и наоборот.