Sentence:  Definition 15.11 (Width Ideal) "An <WSD>ideal</WSD> Q of a poset P = (X,≤) is a width <WSD>ideal</WSD> if maximal(Q) is a width antichain. 
Meaning of the word marked with <WSD> tags:  A non-empty lower set (of a partially ordered set) which is closed under binary suprema (a.k.a. joins).<ref>(Boolean prime ideal theorem#Prime ideal theorems)</ref>