actinoid
1 of 2adjective
ac·ti·noid ˈak-tə-ˌnȯid 
1
: resembling a ray especially of a radially symmetrical animal
2
: exhibiting radial symmetry
actinoid
2 of 2noun
ac·ti·noid ˈak-tə-ˌnȯid
plural actinoids
chemistry
: actinide
The actinoid elements (or actinides; An) constitute a series of 14 elements which are formed by the progressive filling of the 5f electron shell and follow actinium in the periodic table (atomic numbers 90-103).
Mirtat Bouroushian, Electrochemistry of Metal Chalcogenides, 2010
Mirtat Bouroushian, Electrochemistry of Metal Chalcogenides, 2010
Word History
Etymology
Adjective
actin(ia) + -oid entry 2 (probably after actinoida or actinoidea )
Noun
actin(ide) + -oid entry 1
First Known Use
Adjective
1846, in the meaning defined at sense 1
Noun
1958, in the meaning defined above