ac·tin·i·um 
(ăk-tĭnē-əm)
(ăk-tĭnē-əm)n. Symbol Ac
A radioactive element found in uranium ores, used in equilibrium with its decay products as a source of alpha rays. Its longest lived isotope is Ac-227 with a half-life of 21.77 years. Atomic number 89; melting point 1,050°C; boiling point 3,198°C; specific gravity (calculated) 10.07; valence 3. See Periodic Table.
[From Greek aktīs, aktīn-, ray (from its radioactivity); see nekw-t- in the Appendix of Indo-European roots.]
- Great Bear Lake
- great blue heron
- Great Britain
- great circle
- greatcoat
- Great Dane
- Great Depression
- Great Dismal Swamp
- Great Dismal Swamp
- great divide
- Great Divide
- Great Dividing Range
- greaten
- greater
- Greater Antilles
- greater omentum
- Greater Sunda Islands
- greatest common divisor
- Great Falls
- Great Grimsby
- Great Grimsby
- greathearted
- greatheartedly
- greatheartedness
- great horned owl