Eicosapentaenoic acid (EPA) is an omega-3 fatty acid. In physiological literature, it is given the name 20:5(n-3).
EPA is a polyunsaturated fatty acid (PUFA) that acts as a precursor for prostaglandin-3 (which inhibits platelet aggregation), thromboxane-3, and leukotriene-5 groups (all eicosanoids).
EPA is obtained in the human diet by eating oily fish or fish oil—cod liver, herring, mackerel, salmon, menhaden and sardine. It is also found in human breast milk. It is available from some non-animal sources—including spirulina and microalgae, and also purslane in trace amounts.
The US National Institute of Health's MedlinePlus lists a large number of conditions in which EPA (alone or in concert with other omega-3 sources) is known or thought to be effective. Most of these involve its ability to lower inflammation.