An epistemological construct used to describe knowledge obtained through observational means. One of the two prongs of Hume's Fork, it is also called a posteriori knowledge. Empirical knowledge is generally gained with the senses.
Contrast with a priori knowledge, which consists of things that are relations of ideas, or constructs of the interior mind.
An easy way to remember the difference is in something Immanuel Kant said, "Necessity is a sure sign of the a priori."
For example, although it is terribly unlikely that the sun will refuse to rise tomorrow, it could happen. Therefore, the sun's 'rising' is empirical knowledge, simply because of the possibility that it may not happen, or that it happen some other way. On the other hand, you could not say that a uncle is not someone's brother, since it is inherent in the definition of uncle that they are the brother of someone with a child. Therefore, whether someone is an uncle is empirically observable, but what an uncle is is not.