A quick explanation of two types of certainty that people tend to confuse.

The first is the one we typically mean in natural language. It’s called psychological certainty. It’s a feeling of certainty; A confidence in something. This is the one we’re talking about when we say things like “Are you 100% sure?”. It is possible that someone is 100% psychologically certain that something is true and that the something is actually false. Just think of religious people. Psychological certainty comes in degrees.

The second is epistemic certainty. This is the one that philosophers usually talk about. It’s the inability to be wrong type of certainty. If one is epistemically certain, then one cannot be wrong. So, if one is epistemically certain about something, then that something is not only true but necessarily true; It cannot be false. This type of certainty is also called cartesian (after Descartes) certainty, infallible certainty and absolute certainty. This type of certainty does not come in degrees; Either one is epistemically certain or one is not.

For convenience, it smart to type p-certain and e-certain to distinguish between them.