I have a question about this class:

Code:
class Word {
	public:
		Word();
		...
	private:
		string lemma;
		string category;
};
A word has a lemma and a (grammatical) category, for example "cat" (lemma) is a "substantive" (category).
Another example: "The" is an "article".
If I derive subclasses from this class Word, for example:

Code:
class Article: public Word {
	public:
		Article(string lemma);
	        ....
	private:
		bool definite;
};
Isn't the member "category" of the class "Word" superfluous? Because, if you make an instance of Article, then it will be clear that the category is "article".
On the other hand, if you're just handling items of type Word, then the category can be very relevant.
So my question is: do I have to keep the member category of class Word or is this bad OOP practice?