A bipolar junction transistor (BJT) can operate in several distinct regions depending on the biasing of its base–emitter and base–collector junctions, and each region has a characteristic current–voltage behaviour:

The transition region refers to the narrow range of operating conditions the transistor passes through as it moves from one well-defined state (say, cut-off) to another (say, saturation), such as during the rise or fall time of a switching waveform. In this intermediate zone, the transistor is not behaving according to the clean, proportional I_C = β·I_B relationship of the active region, nor is it fully saturated with a fixed, near-zero V_CE. Instead, both the base–emitter and base–collector junction conditions are changing simultaneously and gradually, so the collector current does not vary in a simple, straight-line (linear) way with the controlling voltage or current — the relationship becomes non-linear, reflecting the gradual, partial switching of the device from one operating state to another.

This rules out the other options: the collector current does not remain constant during this transition, since it is actively changing as the device switches between states (a constant I_C is instead characteristic of the active or saturation regions individually); the transistor is not fully non-conducting throughout the transition, since it is precisely because current is changing (not simply absent) that the region is non-linear rather than "off"; and the response is not perfectly linear, because a perfectly linear I_C–V_CE relationship is characteristic only of the well-behaved active (amplification) region, not of the gradual switching behaviour seen in the transition region.