Definition of booster noun from the Oxford Advanced Learner's Dictionary

booster

noun
ˈbuːstə(r)
 
; ˈbuːstər
 
 
1 (also booster rocket) a rocket that gives a spacecraft extra power when it leaves the earth, or that makes a missile go further2 a device that gives extra power to a piece of electrical equipment3 an extra small amount of a drug that is given to increase the effect of one given earlier, for example to protect you from a disease for longera tetanus booster4 a thing that helps, encourages or improves somebody/somethinga morale/confidence booster5 (especially North American English) a person who gives their support to somebody/something, especially in politicsa meeting of Republican boosters