A "Booster" typically refers to a dose of a vaccine given after the initial series to enhance or restore immunity. This is important because immunity can wane over time, making individuals more susceptible to diseases. Boosters are commonly used for vaccines against illnesses like COVID-19, influenza, and tetanus. In addition to vaccines, the term "booster" can also refer to devices that amplify signals, such as in telecommunications. For example, a cell phone booster improves reception in areas with weak signals, allowing for better communication. Both uses of the term emphasize the idea of enhancement or support.