why the resistance applied on the output circuit is called "load resistance" ?

 

Because it makes no sense to call it a source resistance.

 

Theoretically, the circuit is doing something. Its working. Its "hauling its load."

 

Another way to think about the term "load resistance" is that the resistance applied to the output of the circuit represents a "burden" or "load" on the circuit.

The more general term "load impedance" is just a term that has arisen to use when referring to any impedance resistive, capacitive, or inductive that is being driven by the circuit.

The term "source resistance" used in an earlier reply refers to the resistance in series with the signal source. This resistance may not manifest itself as a physical component since it may be present internal to the signal source. However, for the purpose of analyzing the circuit it should be taken into account.

Hope that clarifies things for you.

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