From French effectif, from Latin effectīvus, from efficiō.

• (non-weak vowel) IPA: /ɪˈfɛktɪv/
• (weak vowel) IPA: /əˈfɛktɪv/
• (Australia) IPA: /əˈfektɪv/

effective

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

   Synonyms: efficacious

   The pill is an effective method of birth control.

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.

   Antonyms: 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. (physics, for any effective theory) approximate; Not describing the fundamental dynamic changes in some system as they happen.

• French: efficace
• German: wirksam, effektiv, wirkungsvoll
• Italian: efficace
• Portuguese: eficaz, efetivo
• Russian: эффекти́вный
• Spanish: efectivo, eficaz

• French: efficace, décisif
• German: wirksam
• Portuguese: eficaz
• Russian: результати́вный

• French: efficace
• German: effektiv, wirkungsvoll
• Italian: efficiente
• Portuguese: eficaz
• Russian: эффекти́вный

• French: en vigueur
• German: wirksam, in Kraft
• Portuguese: efetivo
• Russian: де́йствующий

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.