A **chance experiment** is something that happens where the outcome is unknown. For example, if we flip a coin, we don’t know if the result will be a head or a tail. An **outcome** of a chance experiment is something that can happen when you do a chance experiment. For example, when you flip a coin, one possible outcome is that you will get a head. An **event** is a set of one or more outcomes.

We can describe events using these phrases:

- Impossible
- Unlikely
- Equally likely as not
- Likely
- Certain

For example, if you flip a coin:

- It is
*impossible* that the coin will turn into a bottle of ketchup.
- It is
*unlikely* the coin will land on its edge.
- It is
*equally likely as not* that you will get a tail.
- It is
*likely* that you will get a head or a tail.
- It is
*certain* that the coin will land somewhere.

The *probability *of an event is a measure of the likelihood that an event will occur. We will learn more about probabilities in the lessons to come.