目前见过的定义的比较确切的是Yoshua Bengio在ACL2010的一篇paper中关于word representation的定义：
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A word Representation is a mathematical object associated with each word, often a vector. Each dimension's value corresponds to a feature and might even have a semantic or grammatical interpretation, so we call it a word feature.
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准确而又留有余地~~
参见：Word Representations: A Simple and General Method for Semi-Supervised Learning, ACL2010