What is power angle in transmission line system?

Power angle: For a generator, the power angle is the difference between the generator induced voltage and the generator terminal voltage. The value of the power angle is same as the load angle. So, in context of generator, power angle and load angle mean same thing.
For the case of transmission line the power angle is the angle between the angles of the voltages at two different points (bus). The transfer of power between the two points of power system is proportional to the sine of this angle.

Power angle is the angle between a generator’s internal voltage and its terminal voltage, or between the voltages at the source and load points of an electrical transmission line.

Power angle is the angle between  voltage and current, so theoretically it can be defined wherever voltage and current exists. Power angle in transmission lines usually depends on variety of factors such as loads, network parameters, line impedance, etc.
Power angle of loads, (angle between load voltage and load current), however, depends mainly on the load type (resistive, inductive, etc) and parameters.