booster

A booster is a thing that helps make something better or stronger.

When we talk about a vaccine booster shot, it's a medical injection that gives your body extra protection against certain diseases. Usually, people get their first dose of a vaccine when they're young, and then later on in life, they need a booster to keep the immunity going. For example, if you had a flu vaccine as a kid, years later you might need a flu booster shot to stay protected.

In other cases, a booster can be something that helps improve or increase the power of something else. Like in sports, a team might have a special player who is called their "booster" because they help motivate and energize their teammates during tough games.