Step 1: Recall what happens at and below threshold.
In an n-channel enhancement MOSFET, the substrate is p-type. The threshold voltage $V_T$ is the minimum $V_{GS}$ needed to attract enough electrons to the oxide-semiconductor interface to form a conducting inversion layer (channel).
Step 2: Analyze the condition $V_{GS} < V_T$.
When $V_{GS} < V_T$, the positive gate field is too weak to invert the p-type region. Holes near the interface are repelled (depleted) but not enough electrons accumulate to form a channel. The region under the gate is depleted of majority carriers.
Step 3: Name the operating region.
The zone where majority carriers are swept out but no inversion layer exists is called the depletion region. The MOSFET is in the sub-threshold (depletion-like) condition, corresponding to the depletion region. \[ \boxed{\text{Depletion region}} \]