Define catalyst.

Explanation

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

1. **Definition of Catalyst**: A catalyst is a substance that increases the rate of a chemical reaction without being consumed or permanently altered in the process.

2. **Function of a Catalyst**: It alters the rate of a reaction, meaning it can make the reaction happen faster or slower, depending on the type of catalyst used.

3. **Non-consumption**: Importantly, a catalyst does not take part in the chemical reaction itself. This means that while it facilitates the reaction, it remains unchanged and can be used again in future reactions.

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