Define stimulants.

Explanation

**Step-by-Step Text Solution:**

1. **Definition of Stimulants**: Stimulants are a class of drugs that increase or enhance the activity of the central nervous system (CNS).

2. **Mechanism of Action**: These drugs work by speeding up neural activity in the brain, leading to heightened alertness, increased energy, and improved concentration.

3. **Contrast with Depressants**: The term "stimulant" is often contrasted with "depressant." While stimulants increase CNS activity, depressants slow it down.

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