Effective

adj. productive, efficient; in operation; impressive, striking

Effectiveness means the capability of producing an effect. 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).
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Adjective
1. producing or capable of producing an intended result or having a striking effect; "an air-cooled motor was more effective than a witch's broomstick for rapid long-distance transportation"-LewisMumford; "effective teaching methods"; "effective steps toward peace"; "made an effective entrance"; "his complaint proved to be effectual in bringing action"; "an efficacious law"
(synonym) effectual, efficacious
(antonym) ineffective, uneffective, ineffectual
(similar) hard-hitting, trenchant
(see-also) efficacious
(attribute) effectiveness, effectivity, effectualness, effectuality
2. able to accomplish a purpose; functioning effectively; "people who will do nothing unless they get something out of it for themselves are often highly effective persons..."-G.B.Shaw; "effective personnel"; "an efficient secretary"; "the efficient cause of the revolution"
(synonym) efficient
(similar) competent
3. works well as a means or remedy; "an effective reprimand"; "a lotion that is effective in cases of prickly heat"
(similar) efficacious
4. exerting force or influence; "the law is effective immediately"; "a warranty good for two years"; "the law is already in effect (or in force)"
(synonym) good, in effect(p), in force(p)
(similar) operative
5. existing in fact; not theoretical; real; "a decline in the effective demand"; "confused increased equipment and expenditure with the quantity of effective work done"
(similar) actual, existent
6. equipped and ready for service; "the fort was held by about 100 effective soldiers"
(similar) operational
(classification) military, armed forces, armed services, military machine, war machine

adj. effective, positive, real

(n.)
That which produces a given effect; a cause.
  

(n.)
Specie or coin, as distinguished from paper currency; -- a term used in many parts of Europe.
  

(n.)
One who is capable of active service.
  

(a.)
Having the power to produce an effect or effects; producing a decided or decisive effect; efficient; serviceable; operative; as, an effective force, remedy, speech; the effective men in a regiment.