Sentence:  "Is an ancestor of" is a <WSD>transitive</WSD> relation: if Alice is an ancestor of Bob, and Bob is an ancestor of Carol, then Alice is an ancestor of Carol. 
Meaning of the word marked with <WSD> tags:  Having the property that if an element "x" is related to "y" and "y" is related to "z", then "x" is necessarily related to "z".