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1a person or thing that helps increase or promote something, in particular. North American a keen promoter of a person, organization, or cause: [as modifier]:athletic booster clubs [in combination] a source of help or encouragement:job fairs are a great morale booster Medicine a dose of an immunizing agent increasing or renewing the effect of an earlier one. the first stage of a rocket or spacecraft, used to give initial acceleration. a device for increasing electrical voltage or signal strength. 2North American informal a shoplifter.