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English definition of “effective”

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effective

adjective uk   /ɪˈfek.tɪv/ us  

effective adjective (SUCCESSFUL)

B2 successful or achieving the results that you want: It's an extremely effective cure for a headache. The lighting for the production made a very effective use of shadow. She's a very effective teacher.
Opposite
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effective adjective (IN FACT)

[before noun] in fact, although not officially: Although she's not officially our boss, she's in effective control of the office.

effective adjective (IN USE)

If a law or rule becomes effective, it starts to be used: The new laws will become effective next month.
effectiveness
noun [U] uk   /-nəs/ us  
C1 There are doubts about the effectiveness of the new drug (= how successful it is) in treating the disease.
(Definition of effective from the Cambridge Advanced Learners Dictionary & Thesaurus © Cambridge University Press)
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