镤（旧译作鎃）是原子序数为91的一种放射性化学元素，化学符号为Pa，属锕系元素。密度大，外观为银灰色的金属，容易与氧、水蒸汽和无机酸反应。形成的各种化合物通常是氧化价+5、+4、+3或+2。地壳中的平均浓度是通常在几兆分之一，但在一些晶质铀矿的矿床可能达到百万分之一。用于原子能工业，目前主要是从废核燃料中提取。

1913年，法扬斯和奥斯瓦尔德·格林首次发现Pa，命名为Brevium。在1917年至1918年由奥托·哈恩和莉泽·迈特纳发现了镤更稳定的同位素Pa，并命名为为“原锕(proto-actinium)”。最后在1949年IUPAC命名将其命名为“镤(protactinium)”，并确认哈恩和麦特纳为发现者。这个名字源于拉丁文“protos”(意为「第一」)和“actinium”(锕元素)，意义为「锕之父」，因为镤放射性衰变产物即是锕。

Protactinium or protoactinium (former name) is a chemical element with symbol Pa and atomic number 91. It is a dense, silvery-gray metal which readily reacts with oxygen, water vapor and inorganic acids. It forms various chemical compounds where protactinium is usually present in the oxidation state +5, but can also assume +4 and even +2 or +3 states. The average concentrations of protactinium in the Earth's crust is typically on the order of a few parts per trillion, but may reach up to a few parts per million in some uraninite ore deposits. Because of its scarcity, high radioactivity and high toxicity, there are currently no uses for protactinium outside of scientific research, and for this purpose, protactinium is mostly extracted from spent nuclear fuel.