actinium actinism • actinium emanation · · · Info · Definitions · Synonyms · Characters · · actinium · · Text Latin · · Script 8 · · Length 4 · · Syllables Definitions · · · (noun) a radioactive element of the actinide series; found in uranium ores. Synonyms: Ac, atomic number 89. · · Noun Definition 1 Synonyms · · · ac · · Synonym 1 atomic number 89 · · Synonym 2 Characters · · a latin small letter a U+0061 c latin small letter c U+0063 i latin small letter i U+0069 m latin small letter m U+006D n latin small letter n U+006E t latin small letter t U+0074 u latin small letter u U+0075