The Collaborative International Dictionary of English v.0.48:

Protactinium \Protactinium\
   The radioactive chemical element, of atomic number 91; Atomic
   symbol Pa; at. wt. of longest-lived isotope, 231 (T1/2 =
   32,500 yrs.) Also called brevium, Uranium X2 and UX2.
   [PJC]

WordNet (r) 3.0 (2006):

protactinium
    n 1: a short-lived radioactive metallic element formed from
         uranium and disintegrating into actinium and then into lead
         [syn: protactinium, protoactinium, Pa, atomic number
         91]

The Elements (07Nov00):

protactinium
Symbol: Pa
Atomic number: 91
Atomic weight: (231)
Radioactive metallic element, belongs to the actinoids. The most stable
isotope, Pa-231 has a half-life of 2.43*10^4 years. At least 10 other
radioactive isotopes are known. No practical applications are known.
Discovered in 1917 by Lise Meitner and Otto Hahn.