[[t]ɪˈfɛk tɪv[/t]] adj.
1) adequate to accomplish a purpose; producing the intended or expected result:
effective teaching methods[/ex]
2) in operation or in force; functioning; operative:
The law becomes effective at midnight[/ex]
3) producing a deep or vivid impression; striking:
an effective photograph[/ex]
4) mil prepared and available for service, esp. military service
5) mil a member of the armed forces fit for duty or active service
Etymology: 1350–1400; ME < L effectīvus=effect(us), ptp. of efficere (see effect) +-īvus -ive ef•fec′tive•ly, adv. ef•fec′tive•ness, ef•fec•tiv′i•ty, n. syn: effective, effectual, efficacious, efficient refer to that which produces or is able to produce an effect. effective is applied to something that produces a desired or expected effect, often a lasting one: an effective speech. effectual usu. refers to something that produces a decisive outcome or result: an effectual settlement. efficacious refers to something capable of achieving a certain end or purpose: an efficacious remedy. efficient, usu. used of a person, implies skillful accomplishment of a purpose with little waste of effort: an efficient manager.

From formal English to slang. 2014.