Why BJT called bipolar?

A Bipolar Junction Transistor (BJT) is called "bipolar" because it uses both types of charge carriers—electrons and holes—for its operation. In an NPN transistor, electrons are the majority carriers in the n-type regions, while holes are the majority carriers in the p-type region. The interaction between these two types of carriers allows the BJT to amplify current. This dual carrier mechanism is what distinguishes BJTs from unipolar devices, like MOSFETs, which rely on only one type of charge carrier.