noun

• (aerospace) The first stage of a multistage rocket, providing primary or supplemental thrust for liftoff and early flight.
• (electrical engineering) A motor-generator set used for voltage regulation in direct current electrical power circuits.
• (immunology) A booster dose.
  "When did you get your last tetanus booster?"
• (explosives) A small quantity of a sensitive explosive that is triggered by a detonator and provides the energy needed to detonate a larger quantity of a less-sensitive explosive.

verb

• (transitive, medicine) To give a booster shot to; to boost.
  ""Hey guys, on the first day of our holiday break I tested positive for Covid. I was vaccinated and boostered which made me lucky enough to only have mild symptoms," he wrote beside a photo that showed him sitting alone inside a testing room wearing a mask."