Skip to Content
Medical Term:


Pronunciation: pro'tak-tin'e-um

Definition: A radioactive element, atomic no. 91, atomic wt. 231.03588, formed in the decay of uranium and thorium; its most long-lived isotope, 231Pa, has a half-life of 32,500 years.

Synonym(s): proactinium, protoactinium

[G. protos, first]

© Copyright 2017 Wolters Kluwer. All Rights Reserved. Review Date: Sep 19, 2016.