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From The Collaborative International Dictionary of English v.0.48:
Ytterbium \Yt*ter"bi*um\, n. [NL., fr. Ytterby, in Sweden. See Erbium.] (Chem.) A rare element of the boron group, sometimes associated with yttrium or other related elements, as in euxenite and gadolinite. Symbol Yb; provisional atomic weight 173.2. Cf. Yttrium. [1913 Webster] Note: Ytterbium is associated with other rare elements, and probably has not been prepared in a pure state. [1913 Webster]