Linear (triode) region: definition

Beginner

An on state where more push across the transistor gives more flow, like water down a steeper hose.

Novice

The region where $V_{\mathrm{GS}}$ is above threshold and $V_{\mathrm{DS}}$ is small (below $V_{\mathrm{GS}} - V_{\mathrm{t}}$ ). The channel stretches all the way from source to drain, and current rises with $V_{\mathrm{DS}}$ almost in proportion, like a resistor whose value the gate controls.

Expert

$V_{\mathrm{GS}} > V_{\mathrm{t}}$ and $V_{\mathrm{DS}} < V_{\mathrm{DSAT}}$ . $I_{\mathrm{D}} = \beta(V_{\mathrm{GS}} - V_{\mathrm{t}} - V_{\mathrm{DS}}/2)V_{\mathrm{DS}}$ in the square law. A switching transistor ends each transition here, and its resistance at small $V_{\mathrm{DS}}$ sets how fast it finishes pulling a node to the rail.