* States of Matter: Matter can exist in three primary states: solid, liquid, and gas.

* Critical Temperature: Each substance has a critical temperature. This is the temperature above which it cannot exist as a liquid, no matter how much pressure is applied.

* Vapor vs. Gas:

* Gas: A gas is a substance that exists in its gaseous state at room temperature and pressure.

* Vapor: A vapor is a gaseous form of a substance that is normally a liquid or solid at room temperature and pressure.

Example:

* Water: At room temperature, water is a liquid. When water boils and becomes a gas, it's called water vapor. This is because water's critical temperature is much higher than room temperature (374 °C).

* Oxygen: Oxygen is a gas at room temperature, so we don't call it "oxygen vapor."

In simpler terms, a vapor is a gas that's trying to be a liquid. It's the gaseous form of a substance that's usually liquid or solid under normal conditions.