effective

Effective is an adjective describing something that succeeds in producing a desired result or outcome. It emphasizes the actual performance or impact rather than merely potential capability. The term is commonly contrasted with efficient, which focuses on the ratio of output to input; something can be effective without being efficient, and vice versa.

From Latin effectivus, derived from efficere "to bring about." The word entered English in the late Middle Ages, gaining broad use in practical contexts such as medicine, law, administration, and rhetoric.

In medicine, effectiveness refers to the real-world performance of a treatment (as opposed to efficacy in controlled trials). In policy and management, effectiveness assesses progress toward stated goals. In law, an "effective" date marks when a rule or contract becomes enforceable, and in administration it can denote the actual impact of programs or regulations.

Assessing effectiveness involves defining objectives, identifying indicators, and measuring outcomes, often using experiments or observational studies. Randomized controlled trials and impact evaluations are common methods. Limitations include confounding factors, context dependence, and time lags between actions and results.

Variants and related terms include effective dose, effectual, effectiveness research, and effective altruism. In everyday language, "effective" can describe strategies, communication, leadership, and tools that reliably achieve intended effects.