1. Having the power to produce a required effect or effects.
    The pill is an effective method of birth control.
  2. Producing a decided or decisive effect.
    The president delivered an effective speech!
  3. Efficient, serviceable, or operative, available for useful work.
    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.
    The curfew is effective at midnight.

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