Ambiguity at a semantic level in NLP is about the meaning of text, for example, "I saw her duck" might mean I have seen her drop down suddenly, or that I saw her pet poultry. Note here how the semantic ambiguity is related to lexical ambiguity of 'her' (as accusative or possessive pronoun) and 'duck' (avoidance action or bird).
Used in Chap. 13: page 189