1. (noun)Thursday, Th the fifth day of the week; the fourth working day Synonyms: thorium, atomic number 90
2. (noun)thorium, Th, atomic number 90 a soft silvery-white tetravalent radioactive metallic element; isotope 232 is used as a power source in nuclear reactors; occurs in thorite and in monazite sands Synonyms: thorium, atomic number 90