A Priori and A Posteriori

The terms "a priori" and "a posteriori" are used in philosophy to distinguish between two different ways of knowing.
"A priori" and "A Posteriori" are also often used to distinguish different types of arguments.

Definition:
- a priori knowledge is independent of experience or experimentation. It is knowledge based on pure reason.

- a posteriori knowledge is dependent on experience or experimentation.

Examples

You know a priori that all bachelors are unmarried; or that cubes have six sides; or that if today is Wednesday then today is not Thursday; that two plus five equals seven, etc.
In all these cases, it suffices that you understand the meaning of the relevant terms for you to gain that knowledge, you don't need to set up an experiment or to observe the world.

You can know a posteriori that some bachelors are beautiful; that Edinburgh is in Scotland; that water is H2O; that pain is correlated to he activation of certain brain areas.
In all these cases, it is not sufficient to sit comfortable in your armchair and reason about the meaning of the relevant terms for you to gain that knowledge. You have to go outside and observe how things are in the world.