The critical temperature of a substance is:

Explanation

**Step-by-Step Solution:**

1. **Definition of Critical Temperature**: The critical temperature of a substance is defined as the temperature above which the vapor of the substance cannot be liquefied, regardless of the pressure applied.

2. **Understanding the Concept**: At temperatures above the critical temperature, the substance will only exist in the gaseous state. This means that no matter how much pressure is exerted on the vapor, it will not condense into a liquid.

3. **Importance of Critical Temperature**: The critical temperature is a crucial property of a substance because it indicates the limits of its phase transitions. It helps in understanding the behavior of gases and liquids under varying temperature and pressure conditions.

4. **Examples**: For instance, the critical temperature of water is 374°C. Above this temperature, water vapor cannot be converted into liquid water, no matter how much pressure is applied.

5. **Conclusion**: In summary, the critical temperature is a fundamental concept in the study of gases and liquids, as it defines the conditions under which a substance can exist as a liquid or gas.

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