In Physics, an effective theory is, similar to a phenomenological theory, a framework intended to explain certain (observed) effects without the claim that the theory correctly models the underlying (unobserved) processes. An example is an effective field theory that "pretends" that certain effects are caused by a field even if it is known that this is not actually the case. In a way, any theory of Physics is fundamentally an effective theory, since there is no meaningful distinction of observables and reality within the scope of Physics (see also FAPP, cogito ergo sum, Phenomenalism, Pragmatism).
The word effective is sometimes used in a quantitative way, "being very or not much effective". However it does not inform on the direction (positive or negative) and the comparison to a standard of the given effect. Efficacy, on the other hand, is the ability to produce a desired amount of the desired effect, or success in achieving a given goal. Contrary to efficiency, the focus of efficacy is the achievement as such, not the resources spent in achieving the desired effect. Therefore, what is effective is not necessarily efficacious, and what is efficacious is not necessarily efficient.
An ordinary way to distinguish among effectiveness, efficacy, and efficiency:
For the medical meaning of 'effectiveness', see discussion at efficacy.