A word vector is the value associated with a word in word2vec. It captures some level of meaning, for example, if WV('dog'), WV('puppy'), WV('cat') and WV('kitten') are the word vectors assocated with 'dog', 'puppy', 'cat' and 'kitten' respectvely, then the vector difference WV('dog')–WV('puppy') will typically be close to WV('cat')–WV('kitten'). Tis makes it possible to asnwre questions such as "dog is to puppy as cat is to what?"
Used on Chap. 13: page 314