capacity or power to produce a desired effect
"concern about the safety and efficacy of the vaccine"
Ability to produce a desired effect under ideal testing conditions.
"The clinical trial confirmed the drug's efficacy in curing the infection, but its real-world performance remains uncertain."
In plain English: Efficacy is how well something actually works to achieve its intended result.
"The new medication showed high efficacy in treating patients with chronic headaches."
Usage: Use efficacy when referring specifically to how well something works in controlled or theoretical situations, distinguishing it from effectiveness which measures real-world results. This term is often used alongside clinical trials and scientific studies to describe the potential power of a treatment or method before broader application.
Efficacy comes from the Old French word efficace, which was borrowed from Late Latin. The original meaning in Latin was "efficacious."