The Collaborative International Dictionary
ittrium
Yttrium \Yt"tri*um\, n. [NL., from Ytterby, in Sweden. See Erbium.] (Chem.) A rare metallic element of the boron-aluminium group, found in gadolinite and other rare minerals, and extracted as a dark gray powder. Symbol Y. Atomic number 39. Atomic weight, 88.9. [Written also ittrium.]
Note: Associated with yttrium are certain rare elements, as erbium, ytterbium, samarium, etc., which are separated in a pure state with great difficulty. They are studied by means of their spark or phosphorescent spectra. Yttrium is now regarded as probably not a simple element, but as a mixture of several substances.
Wiktionary
ittrium
n. (archaic form of yttrium English)