before vowels actin-, word-forming element meaning "pertaining to rays," from Latinized form of Greek aktis (genitive aktinos) "ray of light, beam of light; spoke of a wheel;" of unknown etymology. It is perhaps cognate with Sanskrit aktuh "light, ray," Gothic uhtwo "dawn, daybreak," Lithuanian anksti "early."
word-forming element in chemistry, used to coin element names, from Latin adjectival suffix -ium (neuter of -ius), which formed metal names in Latin (ferrum "iron," aurum "gold," etc.). In late 18c chemists began to pay attention to the naming of their substances with words that indicate their chemical properties. Berzelius in 1811 proposed forming all element names in Modern Latin. As the names of some recently discovered metallic elements already were in Latin form (uranium, chromium, borium, etc.), the pattern of naming metallic elements in -ium or -um was maintained (in cadmium, lithium, plutonium, etc.; helium is an anomaly).
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Definitions of actinium from WordNet
a radioactive element of the actinide series; found in uranium ores;