Efficacy (noun)

1. The ability to produce a desired effect or result.
2. The degree of usefulness or effectiveness of a product, drug, or treatment.
3. The power to produce an intended result or effect.
4. The effectiveness of a thing in producing the intended result.

Origin:

Late middle english: from latin efficacitas, from efficax, efficac- "effective", from ef- "out" + facere "make".

Examples:

1. The efficacy of the new drug was tested on animals.
2. The efficacy of the vaccine was proven in clinical trials.
3. The efficiency of the solar panels was measured in terms of its efficacy.
4. The efficacy of the program was evaluated based on its results.
5. The efficacy of the treatment was determined by measuring the reduction in symptoms.