Stimulant
Imagine struggling to wake up in the morning because your alarm clock is too quiet to effectively get you out of bed. In a similar way, stimulants act like turning up the volume on your alarm, making it easier for your body to "wake up" its nervous system and respond more actively to the environment. Just as a louder alarm increases your alertness and prompts you to start your day, a stimulant increases the activity of your nervous system, enhancing your alertness and energy.
Practice Version
Stimulant: A drug that increases the activity of the nervous system. Stimulant. A stimulant is a substance that speeds up brain activity and boosts energy or alertness.