The (complex) impedance presented to the output port of a device. Load impedance determines output power, linearity, and efficieny. It influences device gain and stability.

For maximum gain (minimum output reflection coefficient), _L = s₂₂'^*
[The load impedance is equal to the conjugate of the output reflection coefficient, modified by any loading from the generator impedance.]

For power, _L is set to present the correct load line to deliver the desired output power.
[_L for power is a function of desired output power, bias point, and device parasitics. It can be approximated by the desired load impedance R_L in parallel with the output capacitance at half operating voltage (C_CB @ V_CE/2), that combination in series with the output inductance (bond wires plus package parasitics). For class C (non-linear operation), R_L ~ (V_CE-V_Csat)²/2P_out. For class A (linear operation), R_L ~ (V_CE-V_Csat)²/7P_out.

See also:
Gain, Generator Impedance, Impedance, Impedance Matching, reflection coefficient, s-parameters