booster

Concise Oxford English Dictionary © 2008 Oxford University Press:
booster/ˈbuːstə(r)/

▶noun

• 1 Medicine a dose of a vaccine increasing or renewing the effect of an earlier one.

• 2 the first stage of a rocket or spacecraft, used to give initial acceleration.

• 3 a source of help or encouragement.

• 4 a device for increasing electrical voltage or signal strength.