Efficacy (n.): Power or capacity to produce a desired effect; effectiveness.
eficácia= (sf.): 1  Qualidade do que é eficaz, capacidade de produzir o efeito desejado ou esperado [ Antôn.: ineficácia.].  2  Capacidade de realização de tarefas com eficiência, com bons resultados
        Effective (n): A soldier or a piece of military equipment that is ready for combat: "The 'company' was no more than two platoons of effectives" (Tom Clancy).
Effective (adj): Having an intended or expected effect. Producing a strong impression or response; striking: gave an effective performance as Othello.Operative; in effect: The law is effective immediately. Existing in fact; actual: a decline in the effective demand. Prepared for use or action, especially in warfare.
effectiveness (n).
        SYNONYMS effective, effectual, efficacious, efficient. These adjectives mean producing or capable of producing a desired effect: an effective reprimand; an effectual complaint; an efficacious remedy; the efficient cause of the revolution.