effective

• eff.

From French effectif, from Latin effectīvus (“productive; effective”), from efficiō (“I make; I bring about”), equivalent to effect +‎ -ive.

• (weak vowel distinction) IPA^(key): /ɪˈfɛktɪv/

• Audio (US):

  (file)

• (weak vowel merger) IPA^(key): /əˈfɛktɪv/
• (General Australian) IPA^(key): /əˈfektɪv/
• Homophone: affective (weak vowel merger)
• Rhymes: -ɛktɪv

effective (comparative more effective, superlative most effective)

1. Having the power to produce a required effect or effects.

   Synonym: efficacious

   The pill is an effective method of birth control.

   • 2016, Justin O. Schmidt, The Sting of the Wild, Johns Hopkins University Press, →ISBN, page 102:

     Fire ants circumvented the problem of an ineffective sting by having an unusual and highly effective venom that when daubed or sprayed on other ants penetrates their waxy protective integumental barrier and kills or disables them.

2. Producing a decided or decisive effect.

   The president delivered an effective speech!

   • 1651, Jer[emy] Taylor, The Rule and Exercises of Holy Living. […], 2nd edition, London: […] Francis Ashe […], →OCLC:

     Whosoever is an effective, real cause of doing his neighbour wrong, is criminal.

3. Efficient, serviceable, or operative, available for useful work.

   Antonym: nominal

   How long does it take to make a bunch of civilians an effective military force?

   My effective income after taxes and child support is $500 a month.

   The effective radiated power is determined by multiplying the transmitter power output with the antenna gain.

   The effective voltage of an alternating current is 0.7 times its peak voltage.

4. Actually in effect.

   effective immediately

   The curfew is effective at midnight.

5. (geometry, of a cycle or divisor) Having no negative coefficients.
6. (algebra, of a group action) Such that no group element acts trivially.
7. (physics, for any effective theory) approximate; Not describing the fundamental dynamic changes in some system as they happen.

Webster's Revised Unabridged Dictionary from 1913 lists efficient and effective as synonyms, but all major dictionaries now show that these words now only have different meanings in careful use. Use of both for the other meaning is however widespread enough that Longman's Exam Dictionary, for example, finds it necessary to proscribe the use of one for the other with several examples at each entry and provides the following summary:

• efficient = working quickly and without waste
• effective = having the desired effect

effective (plural effectives)

1. (military) a soldier fit for duty
   • 1876, Dabney Herndon Maury, Southern Historical Society Papers: Volume 2, Number 4, Recollections of the Elkhorn Campaign:

     The Army of the West reached Corinth sometime after the battle of Shiloh. We were 15,000 effectives, and brought Beauregard's effective force up to 45,000 men.

• IPA^(key): /e.fɛk.tiv/

• Audio:

  (file)

• Homophone: effectives

effective

1. feminine singular of effectif

effectīve

1. vocative masculine singular of effectīvus