The philosophical position that all substantive knowledge derives from experience rather than from innate ideas or pure reason — that the mind brings no content to the world that the world has not first supplied.
Locke's Essay Concerning Human Understanding (1690) set the program by attacking innate ideas and treating the mind as initially unfurnished, then explaining how complex concepts are assembled from simple ideas of sensation and reflection. Berkeley pressed the premise further than Locke wanted, arguing that if all we ever access is experience then matter is an unnecessary posit. Hume drew the conclusions that made the tradition genuinely unsettling: that no observation of past regularities can justify inference to future ones, that the self is a bundle of perceptions rather than a substance, and that no description of how things are entails a conclusion about how they ought to be.
The political consequences are substantial. An empiricist cannot ground rights in self-evidence, which is why the tradition tends toward conventional, utilitarian or experience-based justifications of political authority, and toward a corresponding suspicion of political programs derived from first principles. Hume's skepticism about abstract political reasoning fed directly into Burke and, much later, into Hayek.
Empiricism supplied the working self-image of modern science and dominated Anglophone philosophy for three centuries, culminating in logical positivism's attempt to dismiss as meaningless any statement neither analytic nor empirically verifiable. That program collapsed under its own criterion, which was itself neither, and Quine's attack on the analytic-synthetic distinction in 1951 undid much of the rest. What survives is less a doctrine than a disposition: that claims should answer to evidence, and that the burden falls on whoever says otherwise.