In philosophy, empiricism is a theory of knowledge which asserts the idea that knowledge arises via sensual experience. Empiricism is one of several competing views that predominate in the study of human knowledge, known as epistemology. Empiricism emphasizes the role of experience and evidence, especially sensory perception, in the formation of ideas, over the notion of innate ideas or tradition  in contrast to, for example, rationalism which relies upon reason and can incorporate innate knowledge.
Empiricism then, in the philosophy of science, emphasizes those aspects of scientific knowledge that are closely related to evidence, especially as discovered in experiments. It is a fundamental part of the scientific method that all hypotheses and theories must be tested against observations of the natural world, rather than resting solely on a priori reasoning, intuition, or revelation. Hence, science is considered to be methodologically empirical in nature.
Etymology and usage
The term "empiricism" has a dual etymology. It comes from the Greek word ἐμπειρία, which translates to the Latin experientia, from which we derive the word experience. It also derives from a more specific classical Greek and Roman usage of empiric, referring to a physician whose skill derives from practical experience as opposed to instruction in theory.