noun

efficacy

EF-i-kuh-see/ˈefɪkəsi/

The ability to produce a desired result.

The filter’s efficacy is shown by the clear water emerging from it.

A working water filter visibly turns muddy incoming water into clear outgoing water, demonstrating its ability to achieve the intended result.

Etymology

efLatinout

+ficLatinmake, do

+acyLatinquality or state

“Out” and “make” combine in Latin efficere, meaning to work out or accomplish. Efficacy is therefore the capacity to carry something through to its intended result, rather than merely to act.

In context

Moconomy17:41

Then we simulate the technical process and calculate its efficacy. Our task at the technological institute is to work out various alternatives to arrive finally at the optimum solution both technically and in terms of cost.

El Hierro - Islands of the Future | When Scarcity Forces Energy Innovation

More examples

There was little evidence of the treatment’s efficacy.
The trial compared the efficacy of two approaches.
Its efficacy depends on consistent application.

Forms

efficacy

efficacious adjective — producing the intended result

Similar words

effectiveness: Effectiveness is the everyday near-equivalent; efficacy often appears in formal evaluations of results.
efficiency: Efficiency concerns resources used; efficacy concerns whether the intended result is achieved.