EPISTEMOLOGY

The branch of philosophy that investigates the nature, sources, limitations, and validity of knowledge.
(Manuel Velasquez & Vincent Barry, Philosophy a Text with Readings, Glossary, p.442)

Branch of philosophy that attempts to determine the nature of knowing, or what the human mind can legitimately hope to discover about the objective world. The study of how the mind knows what it knows.
(Morris Berman, The Reenchantment of the World, p.345)

Epistemology is the science that tells a fallible, conceptual consciousness what rules to follow in order to gain knowledge of an independent reality. Without such a science, none of [a hu]man's conclusions, on any subject, could be regarded as fully validated. There would be no answer to the question: how do you know?
(Leonard Peikoff, Objectivism: The Philosophy of Ayn Rand, p.38)

Epistemology, of course, is a branch of philosophy; it is the science that studies the nature and means of human knowledge. Its primary purpose is to establish the criteria of knowledge, to define principles of evidence and proof, to enable [a hu]man to distinguish between that which [she or] he may and may not regard as knowledge.
(Nathaniel Branden, The Psychology of Self-Esteem, p.98)