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Archaeological lead refers to the element, lead (Pb), found at archaeological sites that was smelted in ancient times. Such lead has less than usual radioactivity, making it highly useful for some types of physics experiments.
Lead is a good (nuclear) radiation shield, and physics experiments sensitive to such radiation are often shielded with it. However, lead, as it is mined, includes a radioactive isotope, and though smelting (i.e., extracting the metal from mined ore) separates lead from other elements, it does not separate individual lead isotopes, and all lead is slightly radioactive immediately after it is mined and smelted. But the radioactive isotope has a 22-year half life, and after a thousand years, it is virtually eliminated, making the lead ideal for some sensitive physics experiments. In particular, experiments that aim to catch the signs of some extremely rare particle-interaction can be overwhelmed by radioactive decay, which may well continually trigger the sensors.
Some of the Mediterranean region's ancient (Roman-era) shipwrecks include lead smelted in the Roman era, used as anchors, and sometimes as a cargo of lead ingots (metal formed in bars, etc., for shipping and storage). This is invaluable to physics, but from an archaeological point of view, such artifacts should be reserved for archaeological study. In at least one case, a deal was made to divide the huge amount of lead from a shipwreck, some for physics, saving most for archaeology. However debate continues about whether that much, or any of such lead should be used for something other than history and archaeology.